Antigen Antigen is a substance which binds specifically with the products (antibodies, T-cells) of the immune system. Its ability to bind with antibodies is called antigenicity. Immunogen It is a substance which produces an immune response as well as binds to its products. So, immunogen is an antigen as well but antigen need not be immunogen. The property of producing an immune response is called immunogenicity.

1. Antigen &
Antigenicity
Amandeep Singh
Assistant Professor
Department of Biotechnology
GSSDGS Khalsa College, Patiala

2. Introduction
Antigen
Antigen is a substance which binds specifically with the products (antibodies, T-cells) of the
immune system.
Its ability to bind with antibodies is called antigenicity.
Immunogen
It is a substance which produces an immune response as well as binds to its products.
So, immunogen is an antigen as well but antigen need not be immunogen.
The property of producing an immune response is called immunogenicity.
Hapten
A group of substances usually very small in size which do not produce an immune response
by themselves alone, but when combine with a carrier, then hapten-carrier complex induces
an immune response.
Most of the drugs producing allergic reactions are haptens,

3. Immunogen & Immunogenicity
Development of an immune response in the host by an
immunogen depends on many factors:
1. Properties/traits of an immunogen
2. Properties/traits of the host
3. Route and dosage of administration of an immunogen
4. Adjuvants

4. Properties/traits of an immunogen
a) Foreignness
b) Chemical nature
c) Molecular size
d) Molecular heterogeneity and structural complexity
e) Susceptibility to antigen processing and presentation

5. Properties/traits of an immunogen
a) Foreignness
Immunogen must be recognized as
foreign/non-self by the host.
More distant taxonomically the immunogen =
More better it is immunogen
For example: BSA (Bovine Serum
Albumin) is more immunogenic to
chicken than to goat.
BSA
More Immunogenic Less Immunogenic

6. Properties/traits of an immunogen
b) Chemical nature
Proteins & Polysaccharides Lipids & Nucleic acids
More Immunogenic
Less Immunogenic
Hapten

7. Properties/traits of an immunogen
c) Molecular size
Immunogen should have a molecular weight of at least 10,000 Daltons to generate an immune response.
5000-10,000 Da = Poor immunogens
1,50,000 Da = Best immunogen
Greater MW = More potent molecule as immunogen.
Insulin (MW 5700 Da) = Poor Immunogen
Glucogon (MW 4600 Da) = Poor Immunogen

8. Properties/traits of an immunogen
d) Molecular heterogeneity and structural complexity
• Simple homopolymers lack immunogenicity regardless
of their size.
• For this reason, synthetic polymer (Teflon) is used to
make artificial implants like heart valves.
• Homopolymers of single amino acids or sugars lack
immunogenicity.
• Polymers of two or more amino acids = Immunogenic
• Addition of aromatic amino acids = Increase
immunogenicity

9. Properties/traits of an immunogen
e) Susceptibility to antigen processing and presentation
Processing & Presentation of (a) Exogenous antigen and (b) Endogenous antigen

10. Properties/traits of the host
Age Nutrition Genetic make up
Poor
Immune
System
Poor
Immune
System
Strong
Immune
System
Malnutrition:
Poor
Immune System
MHC genes
BCR genes
TCR genes

11. Dosage of an Immunogen
Optimal DosageToo low Dosage Too large Dosage
Fail to elicit
Immune response
Immunological tolerance
0.5 μg
Purified pneumococcal
Capsular polysaccharide
0.5 mg
Purified pneumococcal
Capsular polysaccharide
Produce
Immune response
Optimal Dose
antibodiesantibodies

12. Dosage of an Immunogen
Repeated immunogen exposure and Booster doses
Single Dose Repeated Dose
No adequate
Immune response
Clonal
Proliferation of
B and T cells
Day 1 In a Month In 3 Months

13. Dosage of an Immunogen
Route of Administration
Intravenous
Intradermal
&
Subcutaneous
Spleen
Lymph node

14. Adjuvants
• These are the substances used to potentiate the action of an immunogen.
• Mixed and Injected with antigen to boost the immune response.
• Example: Alum (Aluminium potassium sulphate) and Freund’s adjuvant
Freund’s adjuvant
Freund’s complete adjuvant Freund’s incomplete adjuvant
Water in Oil Emulsion
Water
Oil
Ag
Killed
Mycobacterium
tuberculosis
Water
Oil
Ag Ag
Water in Oil Emulsion
Mannide
mono-oleate
Emulsifying
agent
Oil in small droplets

15. Action of Adjuvants
• By prolonging antigen exposure
• By activating the phagocytes
• By activating the Helper-T cells
• By granuloma (collection of many immune cells) formation