Introduction
An antigen (Ag) ("antibody generator") is a molecule of endogenous or exogenous origin that is capable of specific binding to an antibody (Ab) or a lymphocyte receptor, and (as an immunogen) of stimulating an immune response. This note covers the characteristics of antigens, epitopes and paratopes, haptens, tolerogens, the types of antigen, and the antibody structures and antibody–antigen interaction that define specificity.
Epitope and paratope
- The epitope is the part of the antigen molecule (5–6 amino acids long) that binds to an antibody (to its paratope) or to a specific lymphocyte receptor. Epitopes are recognised by antibodies, TCR and BCR. Antibodies recognise conformational (discontinuous) epitopes (3–4 amino acids); the TCR recognises linear epitopes.
- The paratope is the antigen-binding part of the antibody; its structure is complementary to the epitope. The fit is like a "key in a lock".
- Interaction is through weak, non-covalent forces: electrostatic, hydrogen bonds, van der Waals forces and hydrophobic interactions.
- Affinity: specific binds one epitope; cross-reactive binds several.