ANTIGENS & IMMUNOGENS

ANTIGENS & IMMUNOGENS
Immunogen:
A substance that induces a specific immune response.
Antigen (Ag):
A substance that reacts with the products of a specific immune response. It may be specifically bound by an antibody (Ab) molecule or T cell receptors (TCR).
There are some Ag do not induce immune response. So all immunogens are antigens, but not all antigens are immunogens .
Hapten:
A substance that is non-immunogenic but which can react with the products of a specific immune response. Haptens are small molecules (MW less than 1000 D such as nickel, chromate, and some drugs) which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule. Free haptens, however, can react with products of the immune response after such products have been elicited. Haptens have the property of antigenicity but not immunogenicity.
FACTORS INFLUENCING IMMUNOGENICITY
A. Factors related to the immunogen:
1. Foreignness: The immune system normally discriminates between self and non-self such that only foreign molecules are immunogenic.
2. Size: There is no absolute size above which a substance will be immunogenic. However, in general, the larger the molecule the more immunogenic it is likely to be. The most potent immunogens are proteins with a MW 100000 D. Substances smaller than 10000 D are not usually immunogenic.