Antigen–Antibody Interactions

An antigen is a substance that can stimulate an immune response and react with an antibody or a sensitized T cell. The capacity of an antigen to elicit an immune response is termed its immunogenicity; its ability to react with an antibody is termed its antigenicity. Low molecular weight substances (e.g., drugs) are not immunogenic by themselves unless combined with a carrier protein. These small molecules are called haptens. Although they cannot evoke an antibody response when injected alone, they can by themselves react with antibody. Immunogenicity is a complex phenomenon that depends not only on the physical properties of the antigen, but also on the biologic system, route of administration, and method of immunization.