ANTIGENS

Immune responses arise as the result if exposure of foreign ‘stimuli’. The compound that evokes the response is referred to either as antigen or immunogen. An immunogen is any infectious agent capable of inducing an immune response. The antigen is any agent capable of binding specifically to lymphocytes/antibodies. All immunogen are antigens but not all antigens are immunogen.
There are three characteristic features that a compound must possess to immunogenic. They are foreignness, chemical complexity and higher molecular weight. Epitopes are the sites either on within the antigen with which the antibody reacts. They are also called as determinant groups or antigenic determinants. Haptens are low molecular weight compounds. They are antigenic and react with immune lymphocytes or antibodies, but they are not immunogenic, e.g. allergic response of some persons to penicillin. Penicillin is a hapten, it can couple with body protein and elicit an immune response.
The binding of antigens with antibodies or immunocompetent cells does not involve covalent bonds.
The binding may involve electrostatic interactions, hydrophobic interactions, hydrogen bonds and Vander Waal’s forces. The strength of the attraction between antigen and antibody is referred to as the affinity. Avidity refers to the strength of interaction between multivalent antigens and antibodies.