The Immune System

The
Immune
System
Definitions
• Immune system - cells, tissues, and molecules that mediate
resistance to infections.
• Immunology - study of structure and function of the
immune system
• Immunity - resistance of a host to pathogens and their toxic
effects
• Immune response - collective and coordinated response to
the introduction of foreign substances in an individual
mediated by the cells and molecules of the immune system
Two types of Immunity
1. Innate (non-adaptive)
• first line of immune response
• relies on mechanisms that exist before infection
2. Acquired (adaptive)
• Second line of response (if innate fails)
• relies on mechanisms that adapt after infection
• handled by T- and B- lymphocytes
• one cell determines one antigenic determinant
Innate Immunity
• Based on genetic make-up
• Relies on already formed components
• Rapid response: within minutes of infection
• Not specific
• same molecules / cells respond to a range of pathogens
• Has no memory
• same response after repeated exposure
• Does not lead to clonal expansion
Adaptive Immunity
• Based upon resistance acquired during life
• Relies on genetic events and cellular growth
• Responds more slowly, over few days
• Is specific
• each cell responds to a single epitope on an antigen
• Has anamnestic memory
• repeated exposure leads to faster, stronger response
• Leads to clonal expansion
Adaptive Immunity
Active and Passive
Active Immunity Passive Immunity
clinical, sub- via breast milk,
Natural
clinical infection placenta
Vaccination:
Live, killed, immune serum,
Artificial
purified antigen immune cells
vaccine
Immunodeficiency
• Loss or inadequate function of various components of the
immune system
• Can occur in any part or state of the immune system
• physical barrier, phagocytes, B lymphocytes, T
lymphocytes, complement, natural killer cells
• The immuno-compromised host
• has an impaired function of immune system
• is at high risk of infection
Immunodeficiency
• Congenital (primary) immunodeficiency
• genetic abnormality
• defect in lymphocyte maturation
• Acquired (secondary) immunodeficiency
• results from infections, nutritional deficiencies or
treatments
• AIDS, chronic leukemia
How the Immune System Works?
• Our immune system is essential for our survival. Without an
immune system, our bodies would be open to attack from
bacteria, viruses, parasites, and more. It is our immune
system that keeps us healthy as we drift through a sea of
pathogens.
• This vast network of cells and tissues is constantly on the

lookout for invaders, and once an enemy is spotted, a
complex attack is mounted.
How the Immune System Works?
• The immune system is spread throughout the body and
involves many types of cells, organs, proteins, and tissues.
Crucially, it can distinguish our tissue from foreign tissue —
self from non-self. Dead and faulty cells are also recognized
and cleared away by the immune system.
• If the immune system encounters a pathogen, for instance, a

bacterium, virus, or parasite, it mounts a so-called immune
response. Later, we will explain how this works, but first, we
will introduce some of the main characters in the immune
system.
White Blood Cells
• White blood cells are also called leukocytes. They circulate
in the body in blood vessels and the lymphatic vessels that
parallel the veins and arteries.
• White blood cells are on constant patrol and looking for

pathogens. When they find a target, they begin to multiply
and send signals out to other cell types to do the same.
White Blood Cells
• Our white blood cells are stored in different places in the
body, which are referred to as lymphoid organs. These
include the following:
• Thymus - a gland between the lungs and just below the
neck.
• Spleen - an organ that filters the blood. It sits in the upper
left of the abdomen.
• Bone marrow - found in the center of the bones, it also
produces red blood cells.
• Lymph nodes - small glands positioned throughout the
body, linked by lymphatic vessels.
There are two main types of leukocyte:
• 1. Phagocytes
• These cells surround and absorb pathogens and break them
down, effectively eating them. There are several types,
including:
• Neutrophils - these are the most common type of phagocyte
and tend to attack bacteria.
• Monocytes - these are the largest type and have several roles.
• Macrophages - these patrol for pathogens and also remove
dead and dying cells.
• Mast cells - they have many jobs, including helping to heal
wounds and defend against pathogens.
There are two main types of leukocyte:
• 2. Lymphocytes
• Lymphocytes help the body to remember previous invaders
and recognize them if they come back to attack again.
• Lymphocytes begin their life in bone marrow. Some stay in
the marrow and develop into B lymphocytes (B cells), others
head to the thymus and become T lymphocytes (T cells).
These two cell types have different roles:
• B lymphocytes - they produce antibodies and help alert the
T lymphocytes.
• T lymphocytes - they destroy compromised cells in the
body and help alert other leukocytes.
How an immune response works
• The immune system needs to be able to tell self from non-
self. It does this by detecting proteins that are found on the
surface of all cells. It learns to ignore its own or self proteins
at an early stage.
• An antigen is any substance that can spark an immune
response.
• In many cases, an antigen is a bacterium, fungus, virus,
toxin, or foreign body. But it can also be one of our own
cells that is faulty or dead. Initially, a range of cell types
works together to recognize the antigen as an invader.
The role of B lymphocytes
• Once B lymphocytes spot the antigen, they begin to secrete
antibodies (antigen is short for "antibody generators").
Antibodies are special proteins that lock on to specific
antigens.
• Each B cell makes one specific antibody. For instance, one
might make an antibody against the bacteria that
cause pneumonia, and another might recognize the common
cold virus.
The role of B lymphocytes
• Antibodies are part of a large family of chemicals called
immunoglobulins, which play many roles in the immune response:
• Immunoglobulin G (IgG) - marks microbes so other cells can
recognize and deal with them.
• IgM - is expert at killing bacteria.
• IgA - congregates in fluids, such as tears and saliva, where it
protects gateways into the body.
• IgE - protects against parasites and is also to blame for allergies.
• IgD - stays bound to B lymphocytes, helping them to start the
immune response.
• Antibodies lock onto the antigen, but they do not kill it, only mark it
for death. The killing is the job of other cells, such as phagocytes.
The role of T lymphocytes
• There are distinct types of T lymphocytes:
• Helper T cells (Th cells) — they coordinate the immune
response. Some communicate with other cells, and some
stimulate B cells to produce more antibodies. Others attract
more T cells or cell-eating phagocytes.
• Killer T cells (cytotoxic T lymphocytes) — as the name
suggests, these T cells attack other cells. They are
particularly useful for fighting viruses. They work by
recognizing small parts of the virus on the outside of
infected cells and destroy the infected cells.

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• This vast network of cells and tissues is constantly on the

• If the immune system encounters a pathogen, for instance, a

• White blood cells are on constant patrol and looking for

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