CHAPTER I: INTRODUCTION VIROLOGY

Introduction

Viruses are particles consisting of protein and nucleic acid (an RNA or DNA genome). Lack
both cellular structure and independent metabolic processes. They replicate solely by exploiting
living cells based on the information in the viral genome.

Definitions

Virology is a branch of microbiology that deals with the study of viruses. Medical
virology therefore, deals with the study of medically important viruses and the diseases they
cause or their effect on human beings.

Viruses are very small entities that can only be examined by electron microscopes. They have
only one nucleic acid: RNA or DNA and can only multiply inside living cells. The deviate the
synthesis machinery of the cell that houses them to direct the synthesized new viral particles,
virions. These virions contain viral genome.

2. Characteristics of viruses

Viruses are pieces of genetic information incapable of autonomous replication packaged in an

independent transmission system. Only this property makes it possible to distinguish the virus
from the plasmids.

Functionally, a virus is a "package" of gene. Chemically, a virus is a particle that contains

nucleic acids inside and a protein coat outside.

3. Differences between viruses and microorganisms

"Microorganisms, although they are small and relatively simple,

i) Are cells and contain double-stranded DNA (DB), the repository of genetic
information, and single-stranded RNA (SB), ribosomal RNA or messenger RNA.

ii) They have their own system of energy production and synthesis of macromolecules.

iii) They divide like all cells, by binary fission.

[Type here]
 iv) With the exception of certain microorganisms such as Chlamydiae and Rickettsiae,

They live an independent life (replication is autonomous). For that reason, bacteria
have about 3000 genes.

Viruses on the other hand are simpler:

(i) They contain only one type of nucleic acid, which can either be the DNA or RNA

(ii) This nucleic acid is small (2-200 genes) unlike in the cell,

(iii) Viral DNAs can be single stranded or double stranded. Also the viral RNA can be
double stranded or single stranded.

They do not have ribosomes, mitochondria, etc. they depend entirely on the host cell for their
protein synthesis and energy production. They are Therefore obligate Intracellular parasites

In some case, the nucleic acid of a viruses is by itself is viable (infectious).

Instead of programming a reproduction by binary fission, the viral genome programs the
synthesis of a very large number of macromolecules, which assemble to form viral particles
(virion). In a single cycle of multiplication, a virus can give rise to hundreds of thousands of
"descendants".

These differences between viruses and microorganisms have three consequences

a) Avirus can persist indefinitely in a host cell by integrating its DNA (or a DNA copy of their
RNA) into the host's DNA.

b) They are difficult to fight because their metabolism is not different from that of the host cell.
It is therefore difficult to find a substance which inhibits the growth of viruses without altering
that of the host cell.

c)Viruses which contain RNA as a genome have a high rate of mutation because of inadequate
corrective system "proofreading" during replication This results in an increased capacity to
change in order to better adapt to the conditions of their growth, ie, to avoid the presence of
antibodies that can neutralized them.

[Type here]
Are viruses are alive?

One view is that inside the host cell, viruses are alive, whereas outside it they are merely
complex assemblages of metabolically inert chemicals. Chemical changes may occur in
extracellular virus particles, but these are in no sense the 'growth' of a living organism. Viruses
are infectious agents with both living and nonliving characteristics.

Living characteristics of viruses:

They reproduce at a fantastic rate, but only in living host cells; they can mutate
Nonliving characteristics of viruses:

They are acellular, that is, they contain no cytoplasm or cellular organelles; they carry out no
metabolism on their own and must replicate using the host cell's metabolic machinery

4. Morphology and Structure of a viruses:

Their structure is generally simple. In the most simplest case, the Virion (viral particles), it
consists of a nucleic acid molecule surrounded by a protein coat (capsid) (Amino acid +
Capsids = nucleocapside or Core)

In more complex cases, the capsid surrounds a nucleus linked to the nucleic acid in the same
way as the histomes. There is most likely a very specific link between the Nucleic acid and the
capsid proteins. The capsid is surrounded by a large number of non-covalently linked
capsomers. Capsomers are visible under an electron microscope, composed of one or more
polypeptides completely enveloping the genome in a "shell" which protects it (from nucleases,
UV lights, etc.)

[Type here]
 Figure1: Structure of a viral particle

The polypeptides of the Capside are may be assembled in a symmetrical way . Basically we have
two types of symmetry are observed which are polyhedral and helicoidal symetry but others
may be have unusual shapes.

a) Polyhedral: here, the capside have many sides. The most common shape is icosahedron, with
20ntriangular faces and 12 corners. Exmples : Poliovirus and herpesvirus

b) Helicoidal symmetry: ribbon-like protein forms spirals around nucleic acide.it may be rigid
or flexible. Example: in Tobacco mosaic virus and Ebola virus

c)Complex shape: here, shapes are unsual example: in Bacteriophage, they have a tail fibre,
sheeths and plates attached to capside and in Poxivirus they have several coats around the
nucleic acid

[Type here]
 Figure 2: Different forms of viral capsides

NB: In some viruses, we sometimes find an envelope which can be bristling with spicules
allowing the virus to attach to cells and to enter or leave them. It exists only in some RNA
viruses. Each capsomer is composed of a single peptide

-The nucleocapsid, rolled up on itself and are surrounded by an envelope of lipoprotein. This
envelope comes from the host cell and is attached to the nucleocapsid in a process of budding.
In most cases, the cytoplasmic envelope, and the formation of particles and their release occurs
at the same time. The envelope can also come from the nuclear membrane (Herpės virus) () or
from the membrane of the endoplasmic reticulum (bunyavirus, hepatitis B virus). In this case,
the viruses bud in the ER and are released when vesicles from the ER fuse with the nuclear
membrane.

[Type here]
-Membrane Envelope lipids belong to the host cell, but host proteins in the envelope are replaced
by viral proteins. In most cases, we find inside the membrane a viral protein, M (for “matrix”),
responsible for the contacts between the viral membrane and the nucleocapsid.

-Proteins outside the membrane, which are always glycoproteins, form spicules also called
peplomers. .

5. Nucleic acid of the virus

a) RNA virus : With the exception of the hepatitis D(Delta) virus which contains a circular
RNA, the other viruses have linear RNA. Only the virus of hepatitis D has no capsid.

With exception to retroviruses that have double stranded RNAs, others have only single stranded
DNAs

b) DNA Viruses: All viruses have a double stranded DNA except the parvovirus which have a
single stranded DNA

6. Viral proteins.

The majority of viral components are proteins whose primary function is to protect the genome
of the virus. They may be internal or external

6.1. External proteins have two main functions:

a) They have a special affinity for receptors on the surface of cells which may be infected. This
explains the specificity of virus-cell interaction. Example. The polio virus can infect only human
and monkey cells.

b) These are antigens that cause the production of antibodies that neutralize the “infectivity” of
the virus. It should be noted that the antibodies against the internal proteins of the virus are not
neutralizing. These antigens are type-specific and vary between members of a group. e.g, Polio
1, 2, 3 (important in vaccines).

6.2) Internal proteins (core)

a) Proteins of the nucleocapsid. In enveloped viruses, there are proteins the of nucleocapsid.
Normally these are group-specific antigens. For example, the N, P and M antigens of influenza
viruses (ie Orthomyxovirus). b) Membrane internal proteins: “Matrix” protein.
[Type here]
c) Enzymes: Sometimes in small quantities, but often very important functionally. For
example, polymerases in negative chain RNA viruses are essential for infectivity.

7. Lipids

In enveloped viruses, membrane lipids comes from the host cell. It is lipids that make viruses
sensitive to lipid solvents such as detergents and bile salt. Lipids are always part of the
envelope, only the smallpox virus contains lipids inside the virus.

8. Stability of viruses

The stability of viruses varies greatly depending on their composition. The most stable viruses
are enteroviruses, such as polioviruses, which are resistant to acidity, lipid solvents and
proteases. Despite this resistance, these viruses are very sensitive to heat: the half-life is counted
in seconds at 60’CTo preserve a specimen, 0’ C is the ideal temperature, because certain viruses,
such as the respiratory syncitial virus, are very sensitive to freezing.

10. Classification of viruses

Viruses are classified based on genomic, structural and epidemiologic characteristics

a) Genomic characteristics:

 DNA or RNA, double stranded (dsDNA or single stranded (ssDNA and dsRNA or
dsRNA )

b) Structural characteristics:

 Capside with or without envelopes

 Symmetry of the capside (polyhedric, helicoidal , complex)

 Size and form

b) Epidemiology characteristics:

 . Enteric virus. Which enters the body and multiplies at the gastrointestinal tract e.g
Picornavirus,

[Type here]
  Respiratory viruses. Which enters the body and multiplies at the respiratory tract e.g
Corona and Picoma

 Arbovirus. Which infects insects that feed on bloods of vertebrates. E.g Tonga, Rhabdo

11. Multiplication of viruses (Life cycle of viruses)

11.1. Fixation or Adsorption: virus bind (specific protein or glycoprotein) on the cell surface.
It only takes place if there are receptors on the surface of the cells. These receptors can be very
specific, eg, - HIV-1 viruses infect T4 lymphocytes (not T8) because its receptor is the marker
for "differentiation. CD4, which is absent in other lymphocytes. -Epstein-Barr virus (herpes)
infects only B lymphocytes, because it uses the complement receptor for its binding, the
complement receptor CR2.

11.2. Penetration: Viruses must cross the cell membrane in order to enter it. They enter either
by direct fussion with cell membrane or by endocytosis through a clathrin coated pit. Enveloped
viruses fuse their envelopes with the cell membranes. They all have a fusion protein in their
envelope that performs this step.

11.3. Uncoating: This is the separation of viral nucleic acid from protein capside. Lysosomal
enzymes, cytoplasmic or viral enzymes may be involved. Example In poliovirus uncoating is
carried out by unidentified enzymes.

11.4 Synthetic phase (Transcription and replication):

The Early Phase involves synthesis of proteins required for replication of viral genetic material
and late phase involves nucleic acid replication and synthesis of capsid and envelope proteins

11.6. Assembly and release of virions: In the case of viruses that do not have an envelope,
assembly is independent of release. The release can take place through two different
mechanisms

 Lysis of the cell: naket viruses leave the cell by rupturing the cell membrane usually
resulting to death of the cell. e.g polio virus

 Buding: enveloped incorporate viral proteins in a specific area of the membrane and bud
through the membrane. The envelope contains the host proteins and lipids. E.g HIV virus

[Type here]
 SOME VIRUSES OF MEDICAL IMPORTANCE

1. DNA Viruses
1.1 Viruses with Single-Stranded DNA Genomes

 Parvoviruses
1.2 Viruses with Double-Stranded DNA Genomes

 Papillomaviruses

 Polyomaviruses

 Adenoviruses

 Herpesviruses
 Poxviruses

 Hepadnaviruses: Hepatitis B Virus and Hepatitis D Virus

2. RNA Viruse
2.1 Viruses with Single-Stranded RNA Genomes, Sense-Strand Orientation
 Picornaviruses
 Astroviruses
 Caliciviruses
 Hepatitis E Virus
 Togaviruses
 Flaviviruses
 Coronaviruses
 Retroviruses
 Human Immune Deficiency Virus (HIV)
2.2 Viruses with Double-Stranded RNA Genomes
 Reoviruses
2.3 Viruses with Single-Stranded RNA Genomes, Antisense-Strand Orientation
 Orthomyxoviruses
 Bunyaviruses

[Type here]
  Arenaviruses
 Paramyxoviruses
 Rhabdoviruses
 Filoviruses (Marburg and Ebola Viruses
3. Subviral Pathogens
 Viroids
 Prions

VIRAL INFECTIONS

Definition: The term “pathogenesis” covers the factors that contribute to the origins and
development of a disease. In the case of viruses, the infection is by a parenteral or mucosal route.
The viruses either replicate at the portal of entry only (local infection) or reach their target organ
hematogenously, lympogenously or by neurogenic spread (generalized infection). In both
cases, viral replication induces degenerative damage. Its extent is determined by the extent
of virus-induced cell destruction and sets the level of disease manifestation. Immunological
responses can contribute to elimination of the viruses by destroying the infected cells, but the
same response may also exacerbate the course of the disease.

 Transmission

Viruses can be transmitted horizontally (within a group of individuals or vertically (from mother
to offspring). Connatal infection is the term used when offsprings are born infected

 Portal of entry

The most important portals of entry for viruses are the mucosa of the respiratory and
gastrointestinal tracts. Intact epidermis presents a barrier to viruses, which can, however, be
overcome through microtraumata (nearly always present) or mechanical inoculation (e.g.,
bloodsucking arthropods).

 Viral dissemination in the organism

[Type here]
There are two forms of infection:

 Local infection

In this form of infection, the viruses spread only from cell to cell. The infection and manifest
disease are thus restricted to the tissues in the immediate vicinity of the portal of entry. Example:
rhinoviruses that reproduce only in the cells of the upper respiratory tract.

 Generalized infection

In this type, the viruses usually replicate to some extent at the portal of entry and are then
disseminated via the lymph ducts or bloodstream and reach their target organ either directly or
after infecting a further organ. When the target organ is reached, viral replication and the
resulting cell destruction become so widespread that clinical symptoms develop. Examples of
such infection courses are seen with enteroviruses that replicate mainly in the intestinal
epithelium, but cause no symptoms there. Clinical symptoms in these infections first arise in the
target organs such as the CNS (polioviruses, echoviruses) or musculature (coxsackie viruses).
 Neurogenic spread

Another mode of viral dissemination in the macroorganism is neurogenic spread along the nerve
tracts, from the portal of entry to the CNS (rabies), or in the opposite direction from the
ganglions where the viruses persist in a latent state to the target organ (herpes simplex)

DIFFERENT TYPES OF VIRAL INFECTION

RABBIES

Rabies is an infectious disease of animals caused by a bullet-shaped, enveloped RNA virus, 180 x 75 nm.
Man is occasionally infected, and once infection is established in the CNS, the outcome is almost
invariablyfatal.

HUMAN RABIES
Is acquired from virus in saliva entering a bite wound caused by an infected animal, usually a rabid dog.

[Type here]
The severity of the bite determines the risk of infection. The disease does not usually spread from man
to man.
Incubation
After inoculation, the virus enters small nerve endings at the site of the bite. The virus slowly travels up
the nerve to reach the CNS where it replicates and then travels down nerves to the salivary glands
where there is further replication. The time it takes to do this depends upon the length of the nerve - a
bite on the foot will have a very much longer incubation period than a bite on the face. The incubation
period may last from two weeks to six months. Very often the primary wound is healed and forgotten by
the time of clinical presentation.
Clinical Presentation
A) Furious Rabies
when the virus reaches the CNS the patient presents with headache, fever, irritability, restlessness and
anxiety. This may progress to muscle pains, salivation and vomiting. After a few days to a week the
patient may experience a stage of excitement and bewracked with painful muscle spasms, triggered
sometimes by swallowing of saliva or water. Hence they drool and learn to fear water (* Hydrophobia).
The patients are also excessively sensitive to air blown on the face. The stage of excitement lasts only a
few days before the patient lapses into coma and death. Once clinical disease manifests, there is a rapid,
relentless progression to invariable death, despite all treatment.
B) Dumb Rabies
Starts in the same way, but instead of progressing into excitement, the subject retreats steadily and
quietly downhill, with some paralysis, to death. Rabies diagnosis may easily be missed.

DIAGNOSIS

Live - difficult diagnosis

- clinical picture, skin biopsy, corneal impression (antibodies only appear very late)
Dead - brain sent to Onderstepoort
" Negri bodies" in cytoplasm of brain cells;
immunofluorescence virus isolation

TREATMENT
1. Wash wound (soap, detergent and water)
2. Anti-rabies serum (human). Passive immunisation.
3. Vaccine (intensive course). Active immunisation.

[Type here]
Examples of rabies causing viruses are HERPESVIRUSES

The family of herpesviruses is very large, and its members infect most animal species.
There are 7 herpesviruses which are known to infect humans:
 Herpes Simplex Virus type 1 and type 2
 Varicella-Zoster Virus
 Cytomegalovirus
 Epstein-Barr Virus
 Human Herpesvirus 6
 Human Herpesvirus 7

DISEASE
It is a characteristic of all herpesviruses that, following primary infection, the virus establishes a
latent infection in the host and may reactivate at any stage. Reactivation is frequently, but not
always, associated with further disease.

Structure of the virion

All herpesviruses are morphologically identical. They have a large double stranded DNA
genome. The VIRION consists of an icosahedral nucleocapsid of about 100 nm in diameter,
which is surrounded by a lipid bilayer envelope. Between the capsid and the envelope is an
amorphous layer of proteins, termed the tegument.

VIRAL GASTRO-ENTERITIS
Paediatric diarrhoea remains one of the major causes of death in young children. This is
especially so in Asia, Africa and Latin America where it causes millions of deaths in the
age group 0-4 years. The main factors for high incidence and mortality are unsafe water or
inadequate
sanitation, requiring social, economic and political solutions. The immediate causes are often of
an infectious nature and include a variety of pathogenic micro-organisms. A range of bacteria
and parasites has been identified = enterotoxigenic Escheritia coli, salmonella, shigella, cholera,
other vibrio bacteria, as well as cryptosporidium, but these account for well below half of
investigated cases. A number of different viruses cause diarrhoea, of which the most important is
the family of ROTAVIRUSES. Rotaviruses have been estimated to cause 30-50% of all cases of

[Type here]
severe diarrhoeal disease in man. Two strains of adenovirus (40 and 41) have also been
associated with diarrhoeal disease. A group of "small round viruses" (discovered by electron
microscopy) have been linked by genetic techniques as closely related to the previously
described "Norwalk" agent, associated with vomiting and diarrhoea.

ROTAVIRUS - (REO virus family)

 Group A subtypes 1, 2, 3, 4 (main human pathogens)
(Further 7 subtypes) also infect animals (monkey, calf, mouse)
 Group B Infects pigs and rats
Found to cause extensive outbreaks in China in past decade
 Group C Infects Pigs (Occasionally Man)
 Group D Infects birds
 Group E Infects pigs
VIRUS MORPHOLOGY:
Particles are 70 nm round, double shelled, enclosing a genome of 11 segments of double
stranded RNA.
The virus is hardy and may even survive in sewage, despite stringent treatment.
Human rota virus has proved difficult to culture in vitro, but the serologically related
monkey and calf rotaviruses grow easily in cell culture.
CLINICAL:
Essentially an ingestion disease (faecal-oral route)
Incubation is short : 1 to 3 days.
Illness: Sudden onset watery diarrhoea, with or without vomiting. May last up to 6 days
(or longer if immunocompromised). The disease is self limiting.
Complications: Dehydration may result, this can be severe and life threatening in young
children.
Treatment: No specific treatment of viral infection is available nor is it really required.
Treatment is aimed at prevention and/or treatment of dehydration by oral and/or
intravenous fluids and electrolytes
Diagnosis: Detection of virus in stools (peaks at day 3 or 4 of diarrhoea):-
1. Latex agglutination

[Type here]
2. Elisa
3. Electron Microscopy (labour intensive, relatively insensitive)
4. Electrophoresis of RNA segments (Antibody can be detected but is not clinically useful)
Prevention:
Nonspecific factors: improved hygiene, education, clean water

Specific - Breast feeding helps to provide passive immunity in the newborn (from
maternal antibodies), Vaccination is still experimental.

EPIDEMIOLOGY
Infection is found world-wide.
All ages can be infected and reinfection can occur (usually asymptomatic).
Age: Infections at < 6 months age and > 5 years of age tend to be asymptomatic and
give degrees of protection against diarrhoeal infection.
Maternity hospitals commonly have resident strains which readily cause asymptomatic
infections of new- borns.
Seasons: In temperate '1st world' populations rota virus is the main cause of winter
gastroenteritis.
In tropical and developing countries, rotavirus diarrhoea occurs all the year round,but with a
peak in summer. However, it is only one of a variety of pathogens causing diarrhoea.
Vaccine: In view of the major role of dehydration from diarrhoea as a cause of childhood
death, the World Health Organisation has waged an intensive campaign for
1. Oral rehydration solutions to prevent or treat dehydration
2. Development of a vaccine for rota virus infection.
No vaccine is currently used routinely, but several candidate vaccines are being evaluated
in children: e.g. animal strains, attenuated human strains, animal-human recombinant
strains, designed to cover all 4 main human pathogenic strains.
The prevention of severe dehydration is the main aim, rather than totally preventing
infection.

EXAMPLE of viruses causing diahrea is

ADENOVIRUS
A limited number of strains of ADENOVIRUS have been causally related to childhood

[Type here]
diarrhoea.
They do not grow in cell cultures and were discovered by Electron Microscopy. ( Recently there
has been limited success in special cell culture systems).
They are classified in the 40/41 serogroup of adenoviruses.
Viruses can be isolated from stools,as well as throat and respiratory secretions.
Adenoviruses in stools can be detected by latex agglutination, and the 40/41 strains can be
detected by specific molecular techniques.
The exact role or significance of these strains in the global picture of childhood diarrhoea,
especially in developing countries, is not yet fully established.
Apart from this 40/41 group, other adenoviruses may be found in the stools of asymptomatic
children.

GASTROENTERITIS IN OLDER CHILDREN AND ADULTS

Apart from the severe problem of diarrhoea in young children there have been outbreaks of
infectious gastro-enteritis in adults. Electron microscopy (of the stools) has revealed two main
groups of virus particles which do not grow in cell culture.
1. Calici viruses (ssRNA) including Norwalk and related agents ('Hawai'; Ditchling; 'W')
2. "small round viruses" about which very little is known NORWALK agent (33nm in size)

Common source' type of explosive outbreaks of gastroenteritis, with limited secondary spread to
household contacts, have been described. These often occur in institutions, or follow common
source ingestion episodes e.g. celebratory feasts.

Vomiting with cramps are more common symptoms than the diarrhoea.
At first, the virus was seldom identified as there were no easy diagnostic tests only expensive
electron microscopy of stools. Serology was limited, as the only antigenavailable was prepared
from known infected stools - not in plentiful supply! Recently, molecular techniques have shown
that many of these agents from different parts of the world are essentially similar. Molecular
techniques have also enabled the expression of viral antigens that can be used in serological
surveys.

For example, a 1993 survey in the UK has shown that Norwalk infection is apparently a common
silent infection in childhood. Antibody prevalence rises to about 16 years of age and then

[Type here]
begins to level off, with 80% of persons above 30 years being seropositive. It was
hitherto believed that Norwalk infections were rare, - it now seems that the disease is
rare, not the infection.

VIRAL RESPIRATORY INFECTIONS

Respiratory infections are common, eg. colds in both adults and children. Most are fairly mild,
self- limiting and confined to the upper respiratory tract (URT). Most are probably viral induced
- at least initially. However, in infants and children, URT infections may spread downwards and
cause more severe infections and even death

Clinical-Anatomical Definitions

Upper Respiratory Tract

1. ColdsMain feature: watery to mucoid, sometimes purulent nasal discharge "coryza". Often
proceeded by a sore throat, sometimes accompanied by fever and often followed by transient
opportunist bacterial infection.

2. Pharyngitis ("sore throat") Generalised erythema of pharynx, not localised to the tonsils and
not associated with coryza. Some fever present

3. Tonsilitis: Local infection of tonsils = red, swollen with exudate on the surface. (Bacterial
tonsillitis is quite common.)

4. Sinusitis & Otitis Media: Painful inflammatory conditions of sinuses and middle ear.
Drainage of these spaces may be impaired and lead to bacterial infection. (Bacterial infections
are usually secondary to viral infection of the nose and pharynx.)
5. Influenza: Fever, myalgia, sore throat, headache, prostration - usually NOT much nasal
discharge compared to a cold. Maybe some cough.

Lower Respiratory Tract

a) Laryngo-Tracheo Bronchitis (Croup): An acute viral inflammation of larynx and trachea in

small children. Often proceeded by a "cold". Accompanied by pyrexia, hoarseness, croaking
cough, stridor, restlessness (respiratory insufficiency). Can be fatal - ie. Life-threatening disease.

[Type here]
b) Acute Bronchitis: Inflammation of bronchi, accompanied by fever, cough, wheezing and
"noisy chest".

c) Acute Bronchiolitis Inflammation of terminal bronchioles in small children. Bronchiole

diameter is larger during inspiration than during expiration and this leads to hyperinflation of air
sacs distal to bronchiole. Complete plugging of bronchiole with air resorption leads to collapse.
These features can be seen on x-ray. These changes cause respiratory embarrassment and can be
life-threatening. Bronchiolitis appears in seasonal epidemics in Britain but it can be seen all the
year round in the Red Cross Childrens' Hospital, in the poorer communities of the Cape
Peninsula. Usually preceeded by coryzal symptoms which later develops into the major
pulmonary illness. Clinically there is fever, rapid respiration, exhausting cough and wheezing.
d) Pneumonia & Bronchopneumonia: Acute respiratory disease accompanied by fever,
restlessness and cyanosis. Often not much clinical "consolidation". Again, can be life
threatening.

CHAPTER 3

INTRODUCTION TO MYCOLOGY AND BIOLOGY OF FUNGI

Introduction:

The term mycology is derived from a Greek word “mykes” meaning mushroom. Therefore
Mycology is the study of fungi and their numerous functions in nature. Hence medical mycology
is the study of medically important fungi. Fungi are prolific and are found nearly everywhere on
earth. They are found in the air, water and soil.

The last three decades have seen unprecedented changes in the pattern of fungal infections in
humans. These diseases have assumed a much greater importance because of their increasing

[Type here]
incidence in persons with the acquired AIDS, in recipients of solid organ or haemopoietic stem
cell transplant (HSCT), in persons with haematological malignancies and in other debilitated or
immune-compromised individuals.

Definition: Fungi are saprophytic eukaryotic organisms that are non-motile, non-
photosynthetic, absorb nutrients from their environment, have cell walls, filamentous structures
and produce spores. They decompose dead organic matter.

The fungi form a separate group of higher organisms, distinct from both plants and animals, in
several major ways. First, fungal cells are encased within a rigid cell wall, mostly composed of
chitin and glucan. This feature contrasts with animals which have no cell wall components.
Secondly, they are heterotrophic and cannot make their organic food as plants, through
photosynthesis. Fungi obtain their nutrients by secreting enzymes for external digestion and by
absorbing the nutrients that are released.

Thirdly, fungi are simpler in structure than plants or animals. There is no division of cells into
organs or tissues. The basic structural unit of fungi is either a chain of tubular, filament-like cells
(termed a hypha) or an independent single cell. Many fungal pathogens of humans and animals
change their growth form during the process of tissue invasion. These dimorphic pathogens
usually change from a multicellular hyphal form in the natural environment to a budding single
celled form in tissue. In most multicellular fungi, the vegetative stage consists of a mass of
branching hyphae, termed mycelium. Each individual hypha has a rigid cell wall and increases in
length as a result of apical growth. In the more primitive fungi, the hyphae remain aseptate
(without cross- walls). In the more advanced groups, however, the hyphae are septate, with more
or less frequent cross- walls. Fungi that exist in the form of microscopic multicellular mycelium
are often called moulds.

Many fungi exist in the form of independent single cells; propagate by budding out similar cells
from their surface. The bud may become detached from the parent cell or it may remain attached
and produce another bud, thereby producing a chain of cells. Fungi that do not produce hyphae,
but simply consists of a loose amount of budding cells are called yeasts.

The continued elongation of the parent cell before it buds, results in a chain of elongated cells,
called pseudohyphae.

[Type here]
There are about 400,000 species but between 100,000 to 200,000 species are well known to man,
depending on how they are classified. About 300 species are known to be pathogenic to man.
Fungi belong to the kingdom of fungi.

II-CHARACTERISTICS OF FUNGI

They have the following features:

 Are eukaryotic micro-organisms- Nucleus bound by a nuclear membrane, endoplasmic

recticulum and mitochondria.
 A cell membrane (plasma lemma) containing glycoproteins, lipids and ergosterol.
 A multi-layered rigid cell wall containing chitin (glucose and mannose).
 Heterotrophict like animals and most bacteria (use organic compounds as a source of
carbon i.e they cannot make their own sugars; no photosynthesis)
 They are chlorophyii deficient and hence cannot synthesis carbohydrate using
carbondioxide, water and sunlight;
 They obtain their nutrients by secreting enzymes for external digestion of dead organic
matters (saprophytes) and absorbing the nutrient that are released.
 They can also live on living organisms are therefore parasites.
 They are non-motile.
 All fungi produce spores and in addition, some produce capsular polysaccharides.
 They cannot tolerate high temperature that bacteria can
 All fungi reproduce asexually and some also sexually.
 The sexual state is called the teleomorph or perfect state. The sexual state (meiotic) state,
where mating types are (+) and (-) are formed. Many fungi have both states, that is sexual
and asexual states. Some species are homothallic and able to form sexual structures
within individual colonies. Most however, are heterothallic and do not form their sexual
structures unless two different mating strains come into contact.
 The asexual state is called the anamorph or imperfect state. Conidia can be large and
complex (macro conidia) or small and simple (micro conidia). When conidia are enclosed

[Type here]
 in a sac, they are termed endospores. Conidia presence, size, shape and location are the
major features used for laboratory identification.
 They are not affected by antibiotics. Humans are usually accidentally infected by
inhalation of spores or inoculation injuries. During infection, the surface components of
the cell walls mediate attachment of the fungus to host cells.
 Yeasts and molds have melanized cell walls, imparting a brown or black pigment. These
are called dematiaceous fungi.
 Vegetative body may be unicellular (yeast) or composed of multicellular threads called
hyphae.
 Sexual reproduction involves the union of two compatible nuclei.
 In the life cycle, plasmogamy means cell fusion while karyogamy is nuclear fusion.
 Sex organs in fungi are called gaemtangia and may be differentiated into male/female
organs that can bear either differentiated sex cells (gametes). If a single mycelium is
capable of reproducing sexually, it is homothallic while when two mycelia are required to
reproduce sexually, it is heterothallic.

III-Nutritional Status of Fungi (Feeding)

They have the following nutritional capabilities:

 Saprophytes: They use non-living organic material for food. They scavenge the
ecosystem that release hydrolytic enzymes for digestion.They recycle carbon, nitrogen
and essential mineral nutrients.
 Parasites: They use organic materials from living organisms and parasitize plants,
animals and human beings.
 Mutualists: They have beneficial relationship with other living organisms, especially
with plants.

IV-Classification of Fungi

Medically important fungi are classified into four phyla, based on the type of sexual
spores produced or types of structure on which or within which the spores are produced.

 Zygomycota: They have the following features;

[Type here]
 1. They have aseptate hyphae.
2. They reproduce asexually through the production of spores that are contained in a
sporangium, while the sexual phase of reproduction is by the production of gametes.
(Zygospores). E.g. Mucor, Rhizopus , Absidia
 Ascomycota
1. They are also called sac fungi.
2. They have septate hyphae.
3. They reproduce asexually through the formation of conidia.
4. Sexual reproduction occurs in a sac called an ascus, resulting in the formation of
ascospores. E. g. Trichophyton, Microsporum, Blastomyces ,Histoplasma.
 Basidiomycota
1. They are also known as club fungi.
2. They have septate hyphae.
3. Asexual reproduction is by the formation of conidia.

The sexual reproduction occurs by extrusion of spores from a club-like structure called a
basidium. E.g. Cryptococcus

 Chytridiomycota

They reproduce sexually by the formation of oospores and asexually through the
formation of spores and conidia

V-Morphological Features

There are three basic morphologies:

 Yeast: It is a unicellular organism that reproduces by budding (blastoconidia formation)

or by binary fission. The continuation of the budding process can produce a chain of
elongated yeast cells called pseudohyphae. Yeasts are larger than bacteria and are
commonly spherical to egg shaped. It is usually a facultative anaerobe and prefers
warmer temperatures. It grows at 370c and forms a creamy opaque or pasty colony on
culture media. It grows rapidly within 24-48hours.

[Type here]
  Mould: It has branching tubular structures called hyphae. It grows at 25 0c. It forms a
mass of hyphae called mycelia on culture media. The colonies formed by moulds are
fluffy, cottony, woolly or powdery. It grows slowly, requiring about 6 weeks growing.
The pattern of branching and the width of the mycelium are aids to the morphological
identification. If the mycelia do not have septa, they are called coenocytic (non-septate).
The terms “hypha”, “mycelium” and “mold” are frequently used interchangeably.
 Dimorphic Fungi: These exist as yeasts or spherules in tissues and exudates at 35-37 0c.
Conversion to the yeast form appears to be essential for pathogenicity. The different
forms are identified by morphological or biochemical characteristics and the appearance
of their fruiting bodies.
 Hyphae.The hyphae may be coenocytic, meaning that they may be aseptate and
multinucleated. They may be also septate. They grow at their tips by apical extension.

VI-Fungal Reproduction

 All fungi reproduce asexually and some also sexually.

 The sexual state is called the telemorph or perfect state.
 The sexual state (meiotic) state, where mating types are (+) and (-) are formed
 Many fungi have both states, that is sexual and asexual states.
 Some species are homothallic (self fertile) and able to form sexual structures within
individual colonies.
 Most however, are heterothallic (mating types) and do not form their sexual structures
unless two different mating strains come into contact
 The asexual state is called the anamorph or imperfect state.
 Fungi reproduce by budding, hyphal extension or spore formation.
 Fungi may reproduce sexually or asexually
 Spores may be either sexual or asexual in origin.
 Sexual reproduction occurs by the fusion of two haploid nuclei (karyogamy), followed by
meiotic division of the diploid nucleus.
 Sexual spores include ascospores, basidiospores, oospores and zygospores, which are
used to determine phylogenetic relationships.

[Type here]
  Asexual reproduction results from division of nuclei by mitosis.
 Asexual spores are produced in sac-like cells called sporangia and are called
sporangiospores.
 Fungi are classified by how they reproduce (sexually or asexually).

1) Asexual Reproduction
Asexual reproduction occurs by elongation of hyphae, budding, or asexual spore production and
through growth from fragments or by formation of asexual spores.
Asexual propagules are termed either spores or conidia depending on their mode of production.
Asexual spores are produced following mitosis whereas sexual spores are produced following
meiosis.
The asexual spores of zygomycetes, which are known as sporangiospores form within sac like
structure known as sporangia. The sporangiospores result from the mitotic cleavage of cytoplasm
in the sporangium. The sporangia are borne on special hyphae called sporangiophore. This
endogenous process of spore formation within a sac is known as sporogenesis
Conidia arise either by budding off conidiogenous hyphae or by differentiation of preformed
hyphae. These develop following mitosis of a parent nucleus and are formed in any manner
except involving cytoplasmic cleavage. This exogenous process is known as conidiogenesis, a
process that occurs both in yeasts and moulds. Conidia are borne on specialised structures called
conidiophore. Conidia production may be blastic or thallic. In blastic development the conidium
begins to enlarge and a septum is formed. Here the conidium originates from part of parent. In
thallic mode of development the conidium is differentiated by a septum before its differentiation.
Thus the conidium results from the conversion of entire parent cell into the conidium. The cell
that gives rise to a conidium is called a conidiogenous cell. Conidiophores are specialised hyphae
that bear conidia or conidiogenous cells. In many cases conidiogenous cells are referred as
phialides
2. Sexual Reproduction
Sexual propagules are produced by the fusion of two nuclei that then generally undergo
meiosis.The first step in sexual methods of reproduction involves plasmogamy (cytoplasmic
fusion of two cells). The second step is karyogamy (fusion of two compatible nuclei), resulting
in production of diploid or zygote nucleus. This is followed by genetic recombination and

[Type here]
meiosis. The resulting four haploid spores are said to be sexual spores, e.g. zygospores,
ascospores and basidiospores. If a sexual spore is produced only by fusion of a nucleus of one
mating type with a nucleus of another mating type (+ and - strains), the fungus is said to be
heterothallic. In contrast, homothallic moulds produce sexual spores following the fusion of two
nuclei from the same strain. For sexual reproduction to occur,two compatible isolates are
required. Zygospores, which are the sexual spores of zygomycetes are round, thick walled
reproductive structures that result from the union of two gamentagia. Ascomycetes produce
sexual spores called ascospores in a special sac like cell known as ascus. In basidiomycetes the
basidiospores are released from basidium, which is the terminal cell of a hyphae

VII-Fungal Spores

They are resistant to heat, cold, acids, bases and other chemicals. They can function as allergens.

They are produced by sexual and asexual means.

1. Asexual spores. There are two types of asexual spores:

 Sporangiospores-These are spores produced within a containing structure called
sporangium. examples are rhizopus and mucor
 Conidia- These are spores borne naked, for example, Aspergillus, Penicillium and
dermatophytes. The asexual spores may be large (macroconidia, chlamydospores) or
small (microconidia, blastospores, arthroconidia).

They may be single or in groups but are produced by specialized hyphae called conidiophores.

2. Sexual Spores: These spores are produced as a result of fusion of two gametes (two
nuclei).There are four main types
 Zygospores: This sexual spore occurs when morphologically identical cells fuse.
 Ascospores: This is produced within a bag-like structure called ascus, which is contained
in a larger structure called a cleistothecium.
 Oospores: This sexual spore occurs when morphologically different cells fuse.
 Basidopores: This is produced by extrusion of club-like structures.

[Type here]
 CHAPTER IV

MYCOTIC INFECTIONS

I-Characteristics of Mycotic Infections

1. Fungi are ubiquitous in nature and most people are exposed to them.
2. The establishment of an infection usually depends on the size of the inoculum and the
resistance of the host.
3. The severity of the disease seems to depend mostly on the immunologic status of the
host.

Therefore, the demonstration of fungi, for an example, in the blood drawn from an intravenous
catheter, can correspond to colonization of the catheter, to transient fungaemia or to a true
infection. The physician must decide which one of these, is the clinical status of the patient. This
decision should be based on clinical parameters, such as the general status of the patient,
laboratory results etc. The decision is not trivial, since the treatment of systemic fungal infections
requires the aggressive use of drugs with considerable toxicity.

Most pathologic fungi do not produce toxins.

1. They show physiologic modifications during a parasitic infection.

2. They are thermo tolerant.
3. They are able to with stand many host defenses and can resist the effects of the active
oxygen radicals (respiratory burst), released during phagocytosis.
4. Mycotic diseases are generally not communicable from person to person.

II-Clinical Manifestations

There are three main groups of clinical syndrome associated with any fungal infection.

1-Mycotoxicosis

This is due to an ingestion of fungal toxins. Most of these infections are accidental. Examples

[Type here]
  Ergot alkaloids of Claviceps purpurea which cause tissue inflammation, necrosis
and gangrene.
 Aflatoxin of Aspergillus flavus which causes liver damage and may be carcinogenic.

2.- Hypersensitivity Reactions/Diseases

This results from the fungal spores in the air. The fungal spore count is one of the indices that are
measured to determine air pollution. It can trigger off asthmatic attack, rhinitis, pneumonitis and
alveolitis.

3-Infections Secondary to Colonization

These infections are classified based on the part of the body that is affected. They are:

 Superficial mycosis
 Cutaneous mycosis
 Subcutaneous mycosis
 Systemic mycosis
 Opportunistic mycosis .This is caused by organisms of low pathogenicity that produce
disease only under conditions of lowered immunity of the host.

III- Specimen collection and laboratory Diagnosis of Fungal Infections

III.1. Specimen collection:

Specimen collection depends on the site affected. Different specimens include hair, skin
scrapings, nail clippings, sputum, blood, CSF, urine, corneal scraping, discharge or pus
from lesions and biopsy.
 All specimens must be transported to the laboratory without any delay to prevent
bacterial overgrowth. In case of delay specimens except skin specimen, blood and CSF
may be refrigerated for a short period.
 Infected hairs may be plucked using forceps. Those hairs that fluoresce under Wood’s
lamp may be selectively plucked. Hairs may be collected in sterilized paper envelopes.
Surface of the skin must be disinfected with spirit before specimen collection. The

[Type here]
 advancing edge of the lesion is scraped with the help of a blunt forceps and collected in
sterilized paper envelopes.
 Discoloured or hyperkeratotic areas of nail may be scraped or diseased nail clipping may
be collected in sterilized paper envelopes.
 Specimens from mucus membranes (oral) must be collected by gentle scraping and
transported to laboratory in sterile tube containing saline. Swabs may be collected from
vagina.
 Corneal scrapings may be collected using a fine needle and inoculated at bedside.
Pus may be collected by aspiration; use of cotton swabs may give false positive
microscopic results.
 Clean catch urine may be collected in a sterile wide-mouthed container.
 Biopsy specimens must be transported in saline.
 In certain cases, pus or exudates must be looked for presence of granules.
III.2. laboratory Diagnosis of Fungal Infections
III.2.1 Microscopy:
Microscopy is used to observe clinical specimens for the presence of fungal elements or to
identify the fungus following culture. It is very simple, easy to perform and sometimes the result
can be diagnostic. Specimens such as pus from a high vaginal swab or other lesions can be
viewed directly on a low power microscope with a slide cover slip. Several specimens are
subjected to KOH mount for direct examination. Specimens from skin scrapings and skin snips
will also be viewed likewise after the keratin have been dissolved by 10% KOH. In the latter
case, lactophenol cotton blue is stain of choice, which stains the fungal elements blue.The
material is mixed with 20% KOH on a slide and a cover slip is placed. The slide is then gently
heated by passing through the flame 2-3 times. The slide is observed on cooling. KOH serves to
digest the protein debris and clears keratinised tissue and increases the visibility. Addition other
stains or dye which may be used include
 Dimethyl sulphoxide (DMSO)
 India Ink (Capsules of Cryptococcus neoformans can be demonstrated by this negative
staining technique)
 Periodic Acid-Schiff (PAS) stain

[Type here]
  Giemsa’s stain (particularly useful in the detection of Histoplamsa capsulatum in the
bone marrow smears), Haematoxylin and Eosin (H&E) stain(Useful for staining tissue
sections).
 Gomori’s methenamine silver nitrate (GMS) stain(essential for tissue pathology)
 Gridley’s stain
 Meyer mucicarmine stain• Gram stain: Candida is best demonstrated in clinical
specimen by Gram stain.
 Masson-Fontana stain is helpful in staining phaeoid (dematiaceous)
III.2.2. Culture: This is required for a definitive diagnosis. Pathogenic fungi are usually grown
on Sabouraud dextrose agar (SDA). It has a slightly acidic Ph (=5.6); cyclohexamide, penicillin,
streptomycin or other inhibitory antibiotics are often added to prevent bacterial contamination
and overgrowth. Each specimen is inoculated on duplicate sets of media and incubated separately
at 25oC and 37oC to reveal dimorphism. Other culture media include potato dextrose agar, corn
meal agar, brain heart infusion, glucose peptone agar and lactophenol blue agar

III.2.3. Serology: This tool may be helpful when it is applied to a specific fungal disease; there
are no screening antigens for fungi in general. Since fungi are poor antigens, the efficacy of
serology varies with different fungal agents. The most common serological tests for fungi are
based on latex agglutination, immunodiffusion, and complement fixation and enzyme
immunoassays. While latex agglutination may favour the detection of IgM antibodies, double.
There are some serologic tests which can detect specific fungal antigens, e.g. cryptococcosis
and histoplasmosis.

Serological testing mostly provides the most rapid means of diagnosing fungal infections.
Majority of these tests are directed at the antibodies produced against the fungus

III.2.4. Antigen detection: It is particularly useful in the diagnosis of cryptococcal meningitis

from CSF specimens. The test is performed by Latex Agglutination or immunodiffusion tests. It
is also helpful in the detection of Aspergillus and Candida antigens in systemic infections

III.2.5. Direct Flourescent Microscopy: This method may be used for identification of fungi,
even on non-viable cultures or on fixed tissue sections. The reagents for this test are presently
difficult to obtain.

[Type here]
III.2.5. Biopsy and Histopathology: A biopsy may be very useful as a source of the tissue-
invading fungi and for the identification of the organism. Usually the Gomori methenamine
silver (GMS) stain is used to reveal the organisms which stain black against a green background.
The H&E stain does not always tint the organism, but it will demonstrate the inflammatory cells.

III.2.6. Molecular Detection of Fungal Infection: The development of fungal nucleic acid
sequences in clinical specimens that are based on PCR. The use of DNA probes are mostly
focused on ribosomal DNA (rDNA) gene complex-18S, 5.8S and 28S genes. These techniques
permit quantification of the amounts of fungal nucleic acid that are present in clinical samples,
thus allowing the microbial load to be measured.

III.2.7. Dermal Hypersensitivity: Delayed hypersensitivity reactions to fungal antigens can be

demonstrated by skin tests. A positive skin does not necessarily indicate an active infection; it
only indicates hence, its value is in epidemiological studies than diagnosis sensitization of the
individual

IV. TYPES MYCOSES

IV.1-SUPERFICIAL MYCOSES

These groups of infections affect the outermost layers of the skin and hair without actually
eliciting host /pathogen hypersensitivity reactions in most cases. Sometimes, the infection can be
so minute that the patient is insensitive of the problem. Example Tinea (Pityriasis and Nigra) and
Piedra (White and Black

IV.1.1. Tinea Nigra Palmaris

This is a chronic infection of the stratum corneum (horny layer of the skin) due to Exophiala
werneckii. Tinea nigra appears light brown to black non-scaly patches mostly on the palm of

[Type here]
hands and fingers. Sometimes, it affects the face and planter aspect of the feet. It is a dimorphic
fungus.

IV.1.2. Pityriasis or tinea versicolor :

It is a Superfcial infection of keratinized cells and is more frequent in areas with moist climates.

The disease: is characterized by hypopigmented spots on the chest/back.

Diagnosis: KOH mount of skin scales: spaghetti and meatballs (bacon and eggs), yeast dusters
and short curved septate hyphae).

Treatment: It can be treated with the use of topical selenium sulfide. Fungemia in premature
infants on intravenous lipid supplements.

VI.2-CUTANEOUS MYCOSES

These are fungal diseases that are confined to the outer layers of the skin, nail or hair. They only
affect the keratinized layers of the body, rarely invading the deeper tissues or viscera. The fungi
involved are called dermatophytes. They are a group of 40 related fungi that belong to three
genera: Microsporum( infects hairs and skin) , Trichophyton( infects the skin and nails) and
Epidermophyton(infects nails and skin) example

IV.2.1. Tinea Corporis

This is small lesions occurring anywhere on the body. It is a disease of childhood. It is also
known as ringworm. It is commonly caused by all species of Dermatophytes

IV.2.2. Tinea Unguium (Onychomycosis)

The infections are on the nails. Sometimes, the infections are lifelong. In the commonest form,
the nail is invaded from the nail bed. Thick, discolored (white, yellow, brown, black), dystrophic
nails. It can occur at any age but is commoner with increasing age. A number of other fungi may
cause onychomycosis including Scopulariopsis brevicaulis, Acremonium species and Fusarium
species.

IV.2.3. Tinea Cruris- “Jock Itch

[Type here]
Infection of the groin, perineum or perianal area. These are erythematous lesions with central
clearing and raised borders in the groin and less commonly, the scrotum. It is usually seen in
young men.

Treatment:

This is generally topical or oral. For infections that are confined to heavily keratinized areas
(palms and soles), keratolytic agents such as whitfield’s ointment (salicylic and benzoic acid
compound) may be effective while for Tinea corporis, T .cruris and T. Pedis, a topical
Ketoconazole 2%, miconazole 2% or clotrimazole 1%, which is applied at the affected part daily
for 2-6weeks will be effective. Extensive or unresponsive infection may require systemic
therapy. Nail infections rarely respond to topical therapy but terbinafine and Itraconazole are
suitable.

IV.3. SUBCUTANEOUS MYCOSES

The fungi that cause subcutaneous mycoses normally reside in soil or on vegetation. They
penetrate the skin to the subcutaneous tissue by traumatic inoculation with contaminated
material. The lesion then becomes granulomatous and expands slowly from the area of
implantation. This may extend into the lymphatics and they are usually confined to the
subcutaneous tissues. However, they may become systemic and produce life-threatening diseases
in rare cases. Example

IV.3.1 Sporothrix schencki

It is a dimorphic fungi. Environmental forms are seen on plant material worldwide as hyphae
with rosettes and sleeves of conidia while tissue forms form cigar-shaped yeast in tissue

Diseases: Sporotrichosis (rose gardener disease) characterized by subcutaneous or

lymphocutaneous lesions. Pulmonary (acute or chronic) sporotrichosis is also common among
Urban alcoholics, particularly homeless (alcoholic rose-garden-sleeper disease).

Treatment: Itraconazole or amphotericin B

IV.4.MUCORMYCOSIS ( ZYGOMYCOSIS)

[Type here]
This is a clinical syndrome caused by a group of fungal species that belong to the order
Mucorales (Zygomycetes). They are spore forming and they grow rapidly in mould forms in both
tissues and the environment. The common species include Rhizopus, Rhizomucor, Absidia,
Cunninghamella and Mucor. Microscopy allows a degree of speciation (e.g. appearance of the
columellae and rhizoids if present).

Pathogenesis

Infection is acquired via the respiratory tract, or in primary cutaneous infection, via the
inoculation of spores into skin abrasions. Spore germination follows in hosts whose immune
response is deficient. Macrophages and neutrophils are important in preventing growth, and
normal human serum is fungistatic. Invasive disease is favoured by hyperglycaemia, acidosis and
in patients receiving desferrioxamine. The cases are not significantly raised in HIV/AIDS
patients as might be expected. Hyphae invade tissues, penetrate blood vessel walls and grow
along the vessel contributing to thrombosis and necrosis.

Rhinocerebral Disease: This is a disease of the immunosuppressed patient and it is common in

the above mentioned associated conditions. The infection leads to septic necrosis and infarction
of the tissues of the nasopharynx and orbit. The patient develops facial pain or headache with
fever and may have orbital cellulitis, proptosis and conjunctival swelling. There may be cranial
nerve defects with a black crusty material that is apparent in the nasopharynx. The invasion of
vessels may lead to retinal artery thrombosis and visual impairment.

Pulmonary Disease

This is usually secondary to neutropenia and seen in BMT or leukemia patients receiving
chemotherapy.

IV.5-SYSTEMIC MYCOSIS

Most of the causative organisms are restricted to a geographic area. The five primary causes of
systemic mycoses are Histoplasmosis, Coccidioidomycosis, Blastomycosis,
Paracoccidioidomycosis and Cryptococcus neoformans. The first four are dimorphic while the
last one is yeast. The fungi that cause Coccidioidomycosis and histoplasmosis exist in nature in

[Type here]
dry soil. Each of these infections is caused by a thermally dimorphic fungus, and most infections
are initiated in the lungs following inhalation of the respective conidia.

General Characteristics

 All the first 3 causes acute pulmonary (asymptomatic or self-resolving in about 95% of
the cases), Chronic pulmonary, or Disseminated infections
 They are inherently virulent.
 All of them exhibit biochemical and morphological features that enable them to evade
host defenses.
 Four of them grow in tissues as budding yeasts while one grows as spherules
(sporangium –like structures filled with endospores).
 The primary focus of infection for all five agents is the lungs.
 Most cases are asymptomatic or mild and of short duration.
 Some resolve without therapy and are accompanied by a high degree of specific
resistance to re-infection.
 In some cases, a secondary spread occurs outside the lungs. It is this secondary spread
that usually brings the patient to the hospital

The spectrum of disease depends on the balance of the virulence of the organism and the host’s
immune competences

Diagnosis: Sputum cytology, Sputum cultures on blood agar and special fungal media
(inhibitory mold agar, Sabouraud) or peripheral blood cultures are useful for Histoplasma since it
circulates in RES cells.

V. 1.Histplasma capsulatm

They are dimorphic fungus. Environmental forms have hyphae with microconidia and
tuberculate macroconidia while tissue forms are small intracellular yeasts with narrow neck on
bud with no capsule and are Facultative intracellular parasite fund in reticuloendothelial (RS)
cells.

[Type here]
Disease: This fungi causes Fungus flu (a pneumonia) characterized with asymptomatic or acute
(but self-resolving) pneumonia with flu-like symptomatology. Hepatosplenomegaly may be
present even in acute pulmonary infections (facultative intracellular RS))

Treatment: Itraconazole for mild, amphotericin B for severe

V.2.Cocidioides immitis

Disease: Valley fever (asymptomatic to self-resolving pneumonia). Desert bumps (erythema

nodosum) and arthritis are generally good prognostic signs and systemic infections are a problem
in AIDS and immunocompromised patients in endemic region (meningitis, mucocutaneous
lesions).
Treatment: Azoles for mild to moderate (itraconazole, etc.), amphotericin B for severe

Disease: they are responsible for Blastomycosis. They are considered less likely to self-resolve
than histoplasma or Coccidioides, so many physicians will treat even acute infections.
Treatment: Itraconazole for mild, amphotericin B for severe

IV.6.OPPORTUNISTIC MYCOTIC INFECTIONS

1. Cryptococcus Neoformans

This is the cause of Cryptococcosis, which is a systemic fungal disease that is caused by a yeast-
like organism.

Cryptococcus neoformans is an encapsulated yeast-like organism that reproduces by budding.

The cell is round or ovoid (4-6 micrometre in diameter), and this is surrounded by a capsule of
variable size. There are four capsular serotypes (A to D).

Transmission: Infection is through the inhalation of aero-solized organisms but there is no

evidence of either person to person transmission or laboratory acquired infection. Rare routes of
transmission include organ transplantation from infected donors or cutaneous inoculation. There
is no zoonotic transmission

Risk Factors:

Those with T-cell defect. Such as AIDS and Post-transplantation (peak period 4-6 weeks)

[Type here]
Pathology:
i. Cryptococcal meningitis-On examination, fever may be absent, there may be minimal or
no nuchal rigidity while, papilloedema, cranial nerve palsy, blindness and seizures occur
late in the infection.
ii. Pulmonary cryptococcosis- it may be asymptomatic or presents with dyspnoea, cough
and chest pain.
iii. Other lesions will include skin lesions, bone lesions, oral lesions, vulvae lesions, post-
transplant pyelonephritis and prostate lesions.

Laboratory Diagnosis:

i. Fungal culture- mostly done with CSF deposit, sputum and urine.
ii. India ink mount (misses 50%) of CSF sediment to find budding yeasts with
capsular "halos
iii. Cryptococcal antigen test- this a latex agglutination test that can be performed on CSF
and serum
iv. Treatment: Drugs used are mostly Amphotericin B and flucytosine for induction,
fluconazole for consolidation and maintenance.
3. Aspergillus fumigatus

It is a monomorphic filamentous fungus with ichotomously branching generaly with acute

angles.

Diseases/Predisposing Conditions:

 Allergic bronchopulmonary asperglosis/asthma, cystic fbrosis (growing in mucous plugs

in the lung but not penetrating the lung tissue)
 Fungus ball: fee in prefrmed lung cavities (surgical removal to reduce
coughing, which may induce pulmonary hemorrhage)
 Invasive asperglosis/severe neutropenia, burns and invades tissues causing infracts and
hemorrhage, Nasal colonization leading to pneumonia or meningitis Celulitis/in burn
patients and may also disseminate

Treatment: Voriconazole for invasive and aspergilloma, glucocorticoids Itraconazole

[Type here]
[Type here]
 CHAPTER 5

INTRODUCTION TO IMMUNOLOGY

Immunology is the study of immunity. Immunity is a complex of physiological defense

reactions which determine the relative constancy of the internal medium of the macro-organism,
hinder the development of the infectious process or intoxication, and are capable of restoring the
impaired functions of the organism.
Historically, the science of immunology originated in the experience that individuals who
recovered Epidemic diseases were resistant and immuned to further attacks of the same disease:
This experience initiate empirical trials to protect people against such diseases. Only when
micro-organisms were recognized as agents responsible for infectious disease was a more
scientific bases available for studies of acquired resistance to infections and development of
different types of vaccines. The principal effectors of the infection are: Antibodies (AB) and
cellular components which had been long known.

Detailed studies carried out clarified the properties of the foreign (non-self) substances called
Antigens which initiate immune responses, destruction of antibodies and their binding with or to
antibodies to form Antigen-antibody complexes, the interaction of these with other humoral
factors (compliments) as well as structure and receptors on their surfaces. It is only recently that
the mechanisms on how the immune system operates or functions and how it is regulated and
controlled have come to be better understand.

The immune response of an organism is its principal physiological adaptive mechanism of

post-defense against non-self-substance. In other words, it main function is guarding and
monitoring the identity of the body.

Unfortunately, under certain conditions it can also produce pathophysiological or immuno-

pathological reactions causing harm to the host. The immunological activities of the organism
are also very important in transplantation and perhaps preventing possible undesirable reactions
have been intensively studied. There is even hope that further studies of the immune system may
help to fight against cancer.

The brief reference with different situations in which immune responses are involved
demonstrates the complexity and broadening scope of immunology, how it has been developed

[Type here]
from attempts to increase immune response example vaccines, some of which have been proven
very effective in order to protect the host against diseases of the immune system (allergies,
Autoimmunity, immunodeficiencies) and attempts to suppress undesirable immune responses
such as prevention of transplant rejection.

It is hoped that the most recent findings will facilitates manipulation of immune responses
according the needs of the organism.

Memory,specificity and recognition of non-self lies at the heart of immunology. We rarely

suffer from a disease twice, such diseases include mumps, measles, chicken pox, whooping
cough. The First contact with an infectious organism or agent clearly imprints source
information, impacts some memory so that the body is effectively prepared to repel any later
invasion by that same organism. These protection is provided by antibodies evolved as a
response to the infectious agent behaving as an antigen. Antigen combination with antibody
leads to elimination of the antigen, following the production of antibody on the first and second
contact with antigen. We can see the basis of immunity for example, when we inject a bacteria
product, such as staphylococcal toxoid into a rabbit several the blood; these reach a peak and
then fall if we now allowed the animal to rest and the events is now dramatically altered. Within
203 days, the antibody level in the blood rises steeply to reach much higher levels than were
observed in the primary response. This secondary response is then characterized by a more rapid
to more abundant production of antibody resulting from the tuning on of the primary antibody
forming system to provide a population of memory cells after first exposure to antigen.

Vaccination utilizes these principles by using a relatively harmless form of the antigen (e.g a
“killed” vaccine). Harmless forms of antigens as primary stimulus to imprint memory. The
body’s defenses are then alerted and any subsequent contact with virulent form of the organism
will lead to a secondary response with an early and explosive production of antibody which will
prevent the infection from taking place.

Specificity as mentioned earlier is a fundamental feature of the Immunological response. The

establishment of memory or immunity of one organism does not confers protection against
another unrelated organism. After an attack of measles we are immuned to further infection but
are susceptible to other agents such as the polio or mumps virus. The body can in fact
differentiate between the 02 organisms.

[Type here]
This ability to recognize one antigen and distinguish each from other goes even further. The
body must also recognized what is foreign i.e. what is non-self. The failure or inability to
discriminate against self and non-self could lead to the synthesis of antibodies directed against
components of the individuals own body (Autoantibodies) which proved to be very
embarrassing.

CHAPTER 6

IMMUNITY

The term immunity has been quite often used as a synonym to resistance, and or on the other
hand associated with the sensitive and specific immune response shown by vertebrates, many
invertebrates can be immuned to infection with various agents and as such, as more general
statement is that an animal demonstrates immunity if it possess or if it has tissues capable of
recognizing and protecting the animal against non-self-invaders. Sometimes, the resistance
conferred by immune mechanisms is not complete. In some case, the host may recover clinically
from an infection and be resistant to certain challenges but some parasite may remain and
reproduce slowly as in toxoplasmosis, Chagas disease and malaria this condition is known as
premunition. In some infection the parasite may elicite and protect against re-infection but the
parasite itself may remain in host unaffected by the immune response as in schistosomiasis. This
condition is known as concomitant immunity.

TYPES OF IMMUNITY

i) Innate, Natural or Non-Specific

ii) Adaptive, Acquired or Specific Immunity

1. Innate, Natural or Non-Specific

Innate immunity is also called Natural or non-specific immunity, it is the resistance that is pre-
existing and is not acquired through contact with a non-self or (foreign entity) organism called
Antigen. It is present from birth and it is the inborn capacity of the body to resist the entering of
[Type here]
micro-organism to the body, if by chance micro-organisms gain entry to the body, innate
immunity eliminates even before development of any disease. It is not acquired through contact
with an organism of antigen. It is non-specific and includes;

i) Resistance of the skin and mucus membrane to infections agents

ii)Phagocytosis of bacteria’s and other invaders by WB and cells of the macrophage

tissue system; Natural killer cells (NK cells)

iii) Inflammation, interferons and a variety of other non-specific factors.

Innate immunity, may vary with age and humoral, or metabolic activity through exercise. Natural
immunity makes the body resistant to same diseases as paralytic virus infection in animals, cattle
plague, and distemper (a viral disease that kills a large percentage of infected dogs).

2. Adaptive, Acquired or Specific Immunity

Acquired immunity is the resistance developed in the body against any specific foreign body or
agent such as bacteria’s, virus, toxins, vaccines or transplanted tissues. These type of immunity is
also known as adaptive immunity it requires time for it development and occurs more frequently
and vigorously (rapid) on secondary response.

There are 02 types of acquired immunity: these 02 basic but closely related types of adaptive
immunity occur in the body. In one of them, the body develops circulating antibodies which are
globulin molecules capable of attacking the invading agent. These type of acquired immunity is
called humoral immunity. Humoral immunity is provided for by activation of B-Lymphocytes,
it is called B-cell Immunity. B-Lymphocytes fight against the invading organisms by secreting
Antibodies into the blood and Lymph. The blood and Lymph are body fluid also called
Humour. As the B-Lymphocytes provides immunity through the humors, these type of
immunity is referred to humoural immunity. It plays an important role in defense mechanism
against bacterial and viral infection.

The second type is acquired through the formation of large number of activated lymphocytes
(mononuclear cells, including T-cells and B-cells) that are specifically designed to destroy the
invading foreign agents. These type of immunity is also called cell mediated or T-cell
immunity. It is the major defense mechanism against viruses, fungi and a few bacterias such as

[Type here]
baccilla, Tobacco bacteria. It is also responsible for delayed allergic reaction, rejection of
transplanted tissues.

2.1. Antibodies (Ab)

An antibody may be defined as a protein produced as a result of the production of antigen. It has
the ability to combine with the antigen that stimulated it production. Antibodies are
immunoglobulins (Ig) produced by plasma cells produced in the presence of antigens (Ag). The
Ig form 20% of the total plasma cells or plasma proteins. The Ab produced by B-Lymphacytes is
found in almost all the tissues of the body.

2.1.1. Classes of antibodies or immunoglobulin

5 classes of antibodies have been identified

 IgG (Ig gama) -

 IgA (Ig alpha)

 IgM (Ig ma)

 IgD (Ig delta) -

 IgE (Ig epsilon) -

Among these Ab groups, Ig-G is a bivalent Ab comprising about 75% of the Abs of the
Normal person.

2.1.2. Structure of Abs

Antibodies are gama globulines called immunoglobulins All the Igs are composed of
combination of light and heavy polypeptide chains. Most are a combination of 02 light and 02
heavy chains. Each heavy chain consist of about 400 Amino acids and each light chain contain of
about 200 amino acid. Each Ab has 02 halves which are identical. The 02 halves are held
together by disulphide bonds. Each half of the antibody consist of one heavy chain (H) and one
light chain (L). The two chain in each half are also joint by disulphide bonds (S-S). The
disulphide bonds allows for the movement gaa chain. In each Ab the light chain (L) is parelle to
one end of the H-chain. The L-chain and part of the H-chain are parallel to each other to form

[Type here]
one arm. The remaining part of the H-chain forms another arm. A hinge joints both arms. Each
chain of the Ab includes 02 regions: constant region and variable region.

2.1.2.1. Constant Region

The amino acid present in these region are similar in number and sequence in all the Abs of each
classes. The region is therefore called constant region or Fc (Fragment crystalline ) region.
The identification and function of different types of Ig depend on the constant region. These
region links to the Ab receptor situated on the surface of the cell membrane. It also causes
compliment fixation. The constant region is also called the compliment binding region.

2.1.2.2. Variable Region

The variable region is smaller compared to the constant region. The amino found in these region
are different in number and placement (sequence) in each Ab and so, it called variable region.
The variable region enables the Abs to recognize the specific Ag and to bind itself with the Ag
because of these, the variable region of the chain is called Antigen-Antibody binding Region
or Fab (Fragment Antigen binding) region.

Structure of an antibody

[Type here]
 2.1.3. Function of Antibodies `````
`````

1- Ig-A is the main principal immunoglobuline in secretions such as milk, saliva, tears
and in secretion of the respiratory tract, intestinal and genital tract.

2- Immunoglobuline-D (Ig D): These Ig acts as an Ag receptor when present on the

surface of certain B-Lymphocytes therefor it is involved in recognition of the Ag by
lymphocytes.

3- Ig-E: The Fc region of IgE binds to a receptor on the surface of mast cells and
basophiles. These bound Ig E to acts as a receptor to the Ag that stimulated it
production. The resulting Ag-Ab complex trigger, allergic responses of the immediate
type hypersensitivity (anaphylactic stock).

4- IgG: It is the predominant Ab in secondary immune responses; it constitutes an

important defense against bacteria and viruses, it is the only Ab that passes through
the placenta and therefor the most important Ig in newborns.

5- IgM: it is the main or principal immunoglobuline produced early during a primary

immune responses. It is the important Ig in Agglutination, compliment fixation and
other Ag-Ab reactions. It is also important in defense against bacteria and viruses. It
can be produced by a feotus with an infection. Since it reaction with antigen can
involve all it 10 binding sites, it has the highest avidity (speed) of all Ig. This is why
the Ig M is said to be pentameric.

2.1.4. Mechanisms of action of antibodies

The Antibodies acts mainly in 02 different ways to protect the body against invading agents:

 By direct attack on invading agent

 By activation of complement system, that then destroys the invader.
a) Direct actions of Ab on invading agents

Takes place when the Ab directly inactivates the foreign organism by any one of the
following method:

[Type here]
 1- Agglutination: In these method, multiple large molecules or particles such as
bacterias or viruses with Ag on their surfaces are bound together, into a clump by the
Abs.

2- Precipitation: In it, the molecular complex of soluble antigens (such as Tetanus

toxins) are converted into insoluble forms and then precipitated.

3- Neutralization: In it, the AB cover the toxic sites of the Antigenic agent.

4- Lysis: Lysis is carried out by the most potent Abs. these Abs directly attacks cell
membranes of invading agents thereby causing rupture of the cell.

b) Action of antibodies through compliment system

The direct action of Ab attacking the Antigenic agent probably under numbered conditions are
not strong enough to play a major role in protecting as body against the invading foreign agent.
Mode of protection comes through the amplifying effect of the compliment system.

The compliment system including serum and membrane bound proteins function in both the
innate and acquired post defense system. The word compliment is a collective word that referred
to the ability of these proteins to compliment with the immune system such as Ab. The principal
proteins in the compliment system are 18 and all these proteins are normally present among the
plasma proteins. Complement proteins are synthesis by the liver and phagocytic cells.

The enzyme precursor of this protein is inactive or dormant but they can be activated in two
separate ways:

- Classical pathway

- Alternate pathway.

 Classical Pathway

[Type here]
The classical pathway is activated by antigen-antibody reaction. Only immunoglobulines-M
(IgM) and IgH activate or fixe compliment via the classical pathway. The antigen-antibody
complex which L1 activates G5 which cleaves C4 and C2 to form C4b2b.

 The Alternate or Alternative Pathway

The compliment system is activated sometimes without the intermediation of an Ag-Ab reaction.
These occur especially in response to large polysaccharides molecules in cell membranes of
some invading agents. These substances react with compliment factors Band D forming an
activation product that activates factors C3, C9 is clearaged and a C3 convertase and is
generated through the actions of B and D and propertin. The Alternate C3 convertase (which is
C3bBb) generates more C3b. These additional C3b combines or binds to the C3 convertase to
form C3bBbC3b, which is the alternate pathway C5 convertase that generates C5b leading to the
production of a membrane attack complex that causes lysis of the cells because the Alternate
pathway does not involve Ag-Ab reaction, it is one of the first line defense against invading
agent capable of functioning even before the

Biological Effects of Compliments

1- Opsonalisation and phagocytosis: One of the products of the compliment fixation,

C3b activates phagocytosis by both Neutrophils and macrophils causing them to
engulf bacteria (phagocytosis) to which the antibody-antigen complex are attached.
These process is called Opsonalisation.

2- Chemotaxis: Fragment C5a causes movement of neutrophils and phagocytosis by

causing large number of this phagocytes to migrate into the local region of the
antigenic agent.

3- Cytocysis: Insertion of the lytic or relative factor C5b789 complex into the cell
surface has a direct effect of rupturing (Lysis) and killing many types of cells,
including erythrocytes, bacterias and humer cells.

4- Agglutination: The compliment product also charge the surface of the invading
organism causing them to adhere to each other, thus agglutination and phagocytosis.

[Type here]
 5- Neutralization of viruses: Fragments of C3aGp9 and C5a of compliment all
activates most cell and basophils causing them to release histamine and several other
substances into plasma. The released substances causes increased local blood flow,
increased leakage of fluid and plasma. That help inactivate or immobilize the
antigenic agent. These same factors plays a major rule in inflammation and allergic
reactions.

6- Inflammation effect: In addition to inflammation effect caused by activation of mast

cells and basophils several other compliment products also contribute to local
inflammation. These product cause the cappilaz leakage of proteins to be increased
and the protein to coagulate, in the tissues space preventing movement of the invading
micro-organism through the tissues.

2.2. Antigens
Normally the immune system recognizes components of the body it protects as “self” and foreign
matter as “non-self”. Antigens (immunogens) provoke highly specific immune response in an
organism.
The name antigen (GK. anti against, genos genus) is given to substances (independent of
chemical nature) which upon injection into the body recognized by immune system of the
organism as a “non-self” and induce formation of immune response:
1. Production of specific antibodies and reacting specifically with them
2. Proliferation and accumulation of sensitive lymphocytes
3. Immune memory
4. Immune tolerance
Antigens are biopolymers which have animal, plant, bacterial, synthetic origin: They are proteins
and different protein complexes in combination with lipids or polysaccharides (lipoproteins,
glycoproteins). Complex polysaccharides and lipopolysaccharides.
Antigens, consequently, are characterized by the following main properties:
1. Antigenicity
2. Specificity
3. Foreignness

[Type here]
4. Immunogenicity and tolerance (Tolerance is specific immunologic unresponsiveness; i.e., an
immune response to a certain antigen does not occur, although the immune system is otherwise
functioning normally. In general, antigens that are present during embryonic life are considered
“self” and do not stimulate an immunologic response)
2.2.1Antigenic determinants (epitopes)
Epitopes are small chemical groups on the antigen molecule that can elicit and react with
antibody. Generally, antibodies recognize and interact with these specific regions antigenic
determinants –epitopes.
The nature of this interaction depends on the size, shape, and chemical structure of the binding
site on the antibody molecule. An antigen can have one or more determinants. Most antigens
have many determinants; i.e., they are multivalent (specificity is depends on surface amino-acid
groups only, not all molecule of biopolymer). Because different determinants are recognized by
different antibodies, the immune system may produce several distinct antibodies against a single
antigen. Antibodies which synthesized for one epitope are named monoclonal.
Antigens have an immunogenic property, especially microbial antigens which can induce
strained immunity
2.2.2 Haptens

Although substances with molecular weight of less than 8000 rarely act as antigens (Ag),
immunity can be developed against substances with low molecular weight in a special way as
follows:

The low molecular weight substance known as hapten first combines with an antigenic substance
such as a protein. These combinations then elicit an immunological response. The Antibodies or
activated lymphocyte that develops against the combination can then react either against the
protein or against the hapten. The haptens that elicit immune responses of these types are usually
drugs, chemical constituents of dust, breakdown products of dandruff from animals, various
industrial chemicals.

[Type here]
2.2.3 Features of Characteristics of Antigens that determines Immunogenicity
The features of antigens that determine immunogenicity in the immune response are as follows:

 Foreignness:

Generally, molecules recognize as self are not immunogenic for, because immunogenicity
molecules must be recognized as non-self.

 Molecular size:

The most potent immune-antigens are usually large proteins generally molecules with a
molecular weight of less than 8 000 are weakly immunogenic. Very small molecules, for
example aa (amino-acids) are non-immunogenic. However, certain small molecules e.g haptens
become immunogenic only when linked to a carrier protein.

 Chemical and structural complexity:

A certain amount of chemical complexity is required for a molecule to express immunogenity e.g
Amino-acid Homopolymeres are less immunogenic than heteropolymeres containing 02 or more
different AA.

 Antigenic Determinants (Epitopes):

[Type here]
The smallest unit of a complex antigen that is capable of binding to an antibody is known as
antigenic determinant, or epitope. An antigen can have 1 or more determinants.

 Genetic constitution of host:

Two strains of the same species of animal may respond differently to the same antigen because
of a different composition of immune response genes.

 Dosage Route and Timing of antigen administration:

Since the degree of immune response depends on the amount of antigen given, the immune
response can be Optimised by carefully defining the dosage, Route of administration, and timing
of administration.

Types of antigens

There are 02 types of antigens:

 Anto or selfantigens: These are antigens present on the body’s own cells
examples are A-antigen and B-antigens on the RBCs.

 Foreign antigens or Non-self antigens: These are antigens that enters our
bodies from outside. Non-self antigens include the receptors of the cell
membranes, viruses, Toxines from microbial organisms, the materials from
transplanted organs or incompatible blood cells, Allergence or allergic substances
like pollen grains and dust particles. The non-self antigens are classified into 02
groups depening on the respond developed against them in the body:

1- The antigens which induce development of immunity or production of

antibodies (immunogenicity).

2- The antigens which react with specific antibodies (allergic reactivity)

immediate hypersensitivity reaction.

3. The Role of lymphocytes in Acquired immunity

Acquired immunity is the product of the body’s lymphocytes system. In individuals with a
genetic lack of lymphocyte or whose lymphocytes have been destroyed by radiation or by
chemical, no acquired immunity can developed, and almost immediately after birth, such an
[Type here]
individual dies from infection. Therefore, as the B- Lymphocytes are therefore essential to the
survival of human beings. In the feotus , lymphocytes develops from the bone marrow and all the
lymphocytes are release in the general circulation. Where they become, differentiated into two
categories.

i) The lymphocytes designed to develop the cellular immunity migrate to the thymus
gland and become transformed into T- lymphocytes.
ii) Lymphocytes designs to develop humoral Immunity are processed in the liver (during
foetal life) and bone marrow (After birth) and are transformed into B-lymphocytes
and released into blood or lymph.

Birth T-lymphocytes and B-lymphocytes are derived originally in the embryo from the
pluripotent haemopoetic stem cells that differentiate and becomes committed to from
lymphocytes. The stem cells are themselves incapable of forming either activated lymphocytes
or antibodies (Abs)

See diagram attached

[Type here]
3.1. B-Lymphocytes2+566%10!

Were first discovered in the bursa of Fabricius in birds. These lymphocytes were sub-sequently
maned as B-lymphocytes. The bursa fabricius is lymphoid organ situated near the cloaca of
birds. These bursa is absent in mammals, and the processing of these B-cells takes place in the
Bone-marrow and liver the B-lymphocytes are responsible for Humoral immunity.

3.1.1.Types of B-lymphocytes

After processing the B-lymphocytes are transformed into 02 types.

1- Plasma cells

2- Memory cells
[Type here]
After this transformation, B-lymphocytes migrate and are stored in a lymphoid tissues of lymph
mode, spleen, bone marrow and the gastro-intestinal tract. The natural plasma cells produces the
antibodies which are gama globulines called immunoglobulins. The rate of the antibody
production is extremely rapid about 2000 molecules per second for each plasma cell. The
antibodies are released into the lyphm and carried to the circulating blood. These process
continuous for several days or week until death of plasma cells the lymphoid.

These memory B-cells occupy the lymphoid tissues throughout the body. The memory are in an
inactive condition until the body is exposed to the same organism on the second time. The
antibodies produced during the second exposure to the foreign antigen are also more potent than
those produced during the first exposure. These phenomenon forms the basic principle of
vaccination against infections or the infection.

3.2.T-Lymphocytes

T-lymphocytes responsible for cell-mediated immunity are processed in the thymus gland, most
of the processing of the T-lymphocytes of the thymus gland occurs shortly before birth of the
baby and for few month after birth. Beyond these period of times removal or surgical removal of
the thymus gland usually will not seriously impair (destroy) the T-lymphocyte Immune system.
However, removal of the thymus gland several months before birth, can completely prevent
development of all cell-mediates immunity. Because it is cell-mediates type of immunity that is
principally responsible for rejection of transplanted organs such as hearts and kidneys, one can
transplant organs with little likelihood of rejection if the thymus is removed from animal, a
reasonable time before birth.

3.2.1. Types and functions of T-lymphocytes ( or T-cells )

During the processing, of T-lymphocytes are transformed into for types:

 Helper T-cells (inducer T-cells) or (Th)

 Cytotoxic T-cells (Killer cells) or NK
 Suppressor T-cells (Ts)
 Memory T-cells (Tm)
The Helper T-cells or T-lymphocytes are the most numerous of the T-cells, usually
constituting more than 75% (¾) of all T-lymphocytes, they serve as major regulator of
[Type here]
 virtually all immune functions. They do these by forming a series of protein mediator
called lymphokines that act on other cells of the immune system as well as on bone
marrow cells. Some of the lymphokines secrets by the Helper T- lymphocytes are the
following:
(i) Interleukin-2
(ii) Interleukin-3
(iii) Interlleukin-4
(iv) Interleukin-5
(v) Interleukin-6

The Helper T-cells mays be divided into two general categories:

(i) Helper-1 ( TH1 ) cells

(ii) Helper-2 (TH2 ) cells.

The TH1 cells are concerned with cellular immunity and secrets two substances:

(a) Interleukin-2 with activates the other T-cells

(b) Gama interferones with stimulates the phagocytic activity of cytotoxic cells,
macrophages, and Natural killer cells (NK ).

The Helper -2 cells are concern with humoral activities and secrets interleukin-4 and
interleukin-5 which are concerned with:

(a) Activation of B-lymphpcytes

(b) Proliferation of plasma cells
(c) Production of Antiodies by plasma cells.

3.2.1.1 Role of cytotoxic T-cells

The cytotoxic T-cells is a direct attack cell, capable of killing micro-organisms and at times some
of the body’s own cells. For these reason, cytotoxic T-cells are frequently called killer-cells.

3.2.1.2. Mechanisms of action of cytotoxic T-cells

1- The outer membrane of cytotoxic T-cells contain some receptor proteins which bind the
antigens to organism tightly with cytotoxic T-cells

[Type here]
 2- The cytotoxic T-cells are enlarged and release cytotoxic substances such as lysosomal
enzymes
3- The substances destroy the invaded organism. In these way each cytotoxic cell can
destroy a large number of invaded organism one after the other.

3.2.1.2. Other functions of cytotoxic T-cells

1-The cytotoxic T-cells also destroys body tissue cells that have been invaded by foreign bodies
particularly virus because many viral particles become entrapped in the membranes of affected
cells. The antigens of the viruses attract the killer cells the cytotoxic cells kill the affected cells
along with the viruses.

2-The cytotoxic cells also destroy cancer cells to had transplant cells or kidney or other types of
cells that are foreign to the person’s own body.

3.2.2. Role of suppressor T-cells

The suppressor T-cells also called regulatory T-cells. These T-lymphocytes are capable of
suppressing the function of both cytotoxic and Helper T-cells. Thus, the suppressor T-cells play
an important roles in preventing the killer T-cells from destroying the body’s own tissues along
with invading micro-organisms. For these reason, the suppressor cells are along with the Helper
T-cells classified as regulatory T-cells.

3.2.2. Role of Memory T-cells

Some of the T-cells activates by an antigen-don’t enter the circulation but remain in lymphoid
tissue (Lymph nock, adenoid, spleen) these T-lymphocytes called memory cells migrates to
various lymphoid tissues throughout the body. When the body is exposed to the same invading
agent for the second time, the memory cells identify the invading agent and immediately activate
the other T-cells leading to eventual destruction of the agent person becomes immunized

[Type here]
HYPER SENSITIVITY

Hypersensitivity or allergy may be defined as acquired abnormal hyper-immuned reaction to Ag

(allergen) during a second or subsequent occasion or contact. Thus, the previous exposure or
contact and the sensitization of the body are essential for allergic reaction. Hypersensitivity
therefore denote condition or circumstance in which an immune response results in exaggerates
or inappropriate reactions that are harmful to the host.

ALLERGENS

Any substance that produces the manifestation of allergy is called an allergen. It may be an Ag
or a protein or any other type of substance. The common types of allergens includes the
following:

1- Food substances such as wheat, egg, milk, chocolate

2- Inhalants for example pollen grains, dust, fungi, perfumes, smoke and disaggreable
oclours.
3- Contactants for example chemical substances, animal and plants
4- Inffections agents like parasites bacterias, viruses and fungi, spirochaetes and rickettsiac
5- Drugs such as Asprin, antibodies like chloroqueen,
6- Physical agents like cold, heat, light, pressure and radiation.

Hypersensitive reactions

There are four main types of immunological hypersensitivity reactions.

1. Types I or immediate hypersensitivity or anaphylactic shock

2. Type II or cytotoxic hypersensitivity
3. Type IV or cell mediates ( Delayed Hypersentivity) reaction
4. Type III or immune complex hypersensitivity
Type I, II , III are anhbody mediated while type IV is cell mediated.

Type I or immediate hypersensitivity are anaphylactic shock

Anaphylacs means exaggerated reactions of the body to an antigen or to any other agent to which
the body already been sensitized. It is also called immediate hypersensitivity reaction because it

[Type here]
develops within few minute of exposure to an antigen (Ag). Anaphylactic reaction are mediated
by IgE and other factors involved in inflammation (inflammation denotes the protection response
of the tissue to the damage or destruction of cells). When the body is exposure to allergens the
Ig-E immunoglobulines are produced. These Ig also called regimes or sensitizing as it binds to
the surface receptors of most cells and circulating basophiles. Most cell are the granulating
wandering cells found in connective tissues and beneath the mucus membranes in the throat,
lungs and eyes. During the subsequent exposure of the body to the same allergen, the allergen
IgE/Ag reaction takes place resulting to degranulation of most cells and the release of chemical
mediators which produce hypersensitivity reaction.

Type II, cytotoxic hypersensitivity

Abs directes at cell surface Ag activates compliments to domage the cells. The Ab ( IgG
and IgM) attaches to the Ag through the fab region and acts as a bridge to compliment via the
Fc region. As a result they may be compliment mediated lysis of the cell as seen in haemolytic
anemiea, ABO transfusion reactions.

Type III, immune complex hypersensitivity

When Ab combines with it specific Ag, immune complexes are formed. These complexes
normally are supposed to be removed but occasionally they persist and are therefore deposited in
the tissue resulting in several disorders in joints causing arthritis, myocarditis (heart), kidney
(nephritis) or blood vessels (vasculitis). Where ever immune complexe are deposited, they
activates the compliment system and polymorphonuclear leucocysts are attracted to the site,
causing inflammation and injury, immune complex diseases, an atrophy ( associate with elevated
IgE levels)

Type IV, cell mediated /delayed hypersensitivity reactions

Cell mediated hypersensibivity is a function of T-lymphocytes and not of AB. It is

transferred by immunologically committed T-cells and not by serum. The result is delayed i.e it
starts hours (days, weeks) after contact with the Ag and often last for some time. Contact
hypersensibivity after sensitization with simple chemicals such as formaldehyde, plant materials,

[Type here]
topically applied cosmetics, soap and drugs. In all cases, small molecules enters the skin and
acting as haptens attached to baby proteins to serve as complete Ags. Cell mediated
hypersensibivity is induced particularly in the skin. When the skin again comes in contact with
the offending agent, the sensitized person develops within, erythema, Necroses of the skin within
12-45 hours.

CYTOKINES

Cytokines are small hormone-like proteins which act as intercellular messengers by binding to
specifics receptors of target cells. These non-Ab protein are secreted by leucocytes and other
types of cells. Their major function is the regulation of the general immune system of the body.
Cytokines are distinct from other cell signaling molecules such as growth factor (GF) and
hormones. Cytokines are soluble mediators of post defense responses both specific and non-
specific. Depending on the source of the secretion and effects, the cytokines are classified into 6
types:

 Interleukins
 Interferones
 Tumor necrosis factor ( INF)
 Chemokins
 Defensis
 And Cathelicidins

Non-specific immunologic mechanism

a) Phagocytosis: During bacterial infection, the number of circulating phagocytic cells often
increases. The main function of phagocytic cells include migration, chemataxis, ingestion and
microbial killing, micro-organism and others particles that enters the lymphatic vessels, lungs,
bone marrow or blood stream are engulf by any of the varieties of the phagocytic cells including
the following:

 Polymorphonuclear leucocytes ( granulocytes)

 Monocytes ( macrophages)
 Fixed macrophages of recticulo-endothelial system ( RES)

[Type here]
 Phagocytosis can occur in the absence of serum Abs. Phagocytic cells are in-effective in
large, smooth, open spaces likes the pleura pericardium or joints but may be more
effective in small tissue spaces like aveoli or un-rough surfaces. Such surface phagotosis
occurs early in the infections process before Abs are available.

Opsonization: Both macrophages and neutrophils have membrane receptors for certain types of
antibodies (IgG) and certain complement components (C3b). If an antigen is coated with either
of these materials, adherence and phagocytosis may be enhanced by up to 4,000-fold. Thus,
antibody and complement are called opsonins, and the means by which they enhance

[Type here]
phagocytosis is called opsonization.

[Type here]