Antigens

The word antigen (Ag) is used in two related senses: a molecule that generates an immune response, and a molecule that reacts with antibodies once that response exists. In general, antigens are large, complex molecules — most commonly proteins, polysaccharides or nucleic acids — capable of provoking a detectable immune response when they are foreign to the recipient; the term is often used interchangeably with immunogen, a substance capable of actually initiating an immune response. Not every antigen-like molecule can trigger a response on its own, though: haptens are small, non-immunogenic substances that can react with the products of an existing immune response but cannot by themselves provoke one. Adjuvants, by contrast, are substances that boost or enhance the immune response mounted against a co-administered antigen.

Two further terms describe the precise contact points in an antigen-antibody encounter. The epitope (or antigenic determinant) is the specific, active portion of an antigen that the immune system actually recognises. The paratope is its mirror image on the antibody side — the antigen-binding region of the antibody molecule that recognises and locks onto that epitope. …