1.4 Antigens

Antigens
EVOCATION
General Objective:
To analyze about antigens, antigenicity and the factors
influencing immunogenicity.
Specific Objectives:
• To infer about antigens and epitopes.
• To attribute the antigenicity and immunogenicity
• To outline the factors influencing immunogenicity
• To infer about antigens and epitopes.
Antigens
• Capable of inducing immune response

• Ig receptor of B cells
• T cell receptors when complexed with MHC
• Types
– Exogenous antigens: Enter into host from
surrounding environments
– Endogenous antigens: Proteins of phagocytic

cells, tumor cells
– Autoantigens: Produced by the host

Epitopes
• Discrete sites on antigens – antigenic

determinants
• Binds to antigen specific receptors or
antibodies (paratope)
• To attribute the antigenicity and immunogenicity
Immunogenicity & antigenicity
• Immunogenicity
– Ability to induce humoral &/or cell mediated immune response
• B cells + antigen = effector B cells (plasma cells) + memory B cells
• T cells + antigen = effector T cells (Tc or Th) + memory T cells
• Antigenicity
– Ability to combine specifically with the final products of the immune
responses (i.e., antibodies and/or cell-surface receptors).
• All molecules that have the property of immunogenicity also have the
property of antigenicity but not vice versa
Hapten
• Antigenic but not immunogenic
• Elicit an immune response only when attached to a large carrier such as
a protein.
• Immune response against,
‐ Hapten
‐ Carrier
‐ Conjugate
Hapten
• To outline the factors influencing immunogenicity
Factors that influence immunogenicity
• Nature of the immunogen

– Foreignness
– Molecular size
– Chemical composition & heterogeneity
– Lipids as antigens
– Susceptibility to antigen processing & presentation
Factors that influence immunogenicity
• Biological system
– Genotype of the recipient animal
– Immunogen dosage & route of administration
– Adjuvants
Nature of the immunogen
• Foreignness
– Non-self – Immune response
– Self antigens – Specific unresponsiveness
– Lymphocytes development – Exposure to self
components
– Degree of immunogenicity depends on the degree
of foreignness
– Depending on the molecule it is considered as
antigen among different species
– Eg: BSA in cow, rabbit, chicken & goat, collagen
and cytochrome c
• Molecular size
– Correlation between size & immunogenicity
– Active immunogens – 100000 Da
– Poor immunogens – 5000 – 10000 Da
– Exceptions - > 1000 Da
• Chemical composition & heterogeneity

– Synthetic homopolymers (single amino acid/sugar) – Lack
immunogenicity
– Copolymers – More immunogenic
– Complexity proportional to immunogenicity
• Lipids as antigens
– Appropriately presented lipoidal antigens
can induce immune response.
– B cells – lipids are used as haptens bound

to carrier keyhole limpet hemocyanin
(KLH) or Bovine serum albumin (BSA).
– T cells – lipoidal compounds (glycolipids &

phospholipids) when presented as
complex with MHC like molecule
(members of CD1 family).
• Susceptibility to antigen processing & presentation

– Macromolecules > small soluble molecules
– Macromolecules are readily phagocytosed and presented
Biological system
• Genotype of the recipient animal

– Type of response & degree of response
– MHC gene products
– Genes that encodes B & T cell receptors
– Genes that encodes proteins involved in

immunoregulatory mechanisms
Biological system
• Immunogen dosage and route of administration

– Some combination of optimal dosage & route of administration will induce a
peak immune response.
– Insufficient dose – No immune response or tolerance
– Excessively high dose – Tolerance
– Single dosage – No strong response
– Boosters – Strong response

Biological system
• Immunogen dosage and route of administration

– Intravenous (iv): into a vein
– Intradermal (id): into the skin
– Subcutaneous (sc): beneath the skin
– Intramuscular (im): into a muscle
– Intraperitoneal (ip): into the peritoneal cavity
– Organs & cells should be involved
– iv – spleen; sc – lymph nodes

Biological system
• Adjuvants
– When mixed with antigens - enhances immunogenicity
– Boost the immune response
– Antigen persistence is prolonged
– Size of the antigen is increased
– Co-stimulatory signals are enhanced
– Local inflammation is increased
– Non-specific proliferation of lymphocytes is stimulated
– Eg: Alum & Freund’s adjuvants; synthetic polyribonucleotides & bacterial
lipopolysaccharides

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Antigens

• Capable of inducing immune response

– Endogenous antigens: Proteins of phagocytic

– Autoantigens: Produced by the host

• Discrete sites on antigens – antigenic

• Nature of the immunogen

• Chemical composition & heterogeneity

– B cells – lipids are used as haptens bound

– T cells – lipoidal compounds (glycolipids &

• Susceptibility to antigen processing & presentation

• Genotype of the recipient animal

– MHC gene products

– Genes that encodes B & T cell receptors

– Genes that encodes proteins involved in

• Immunogen dosage and route of administration

– Insufficient dose – No immune response or tolerance

– Excessively high dose – Tolerance

– Single dosage – No strong response

– Boosters – Strong response

• Immunogen dosage and route of administration

– Organs & cells should be involved

– iv – spleen; sc – lymph nodes

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