foreign substance that, when introduced into the body, is capable of stimulating an immune response, specifically activating lymphocytes (lymphocyte), which are the body's infection-fighting white blood cells. Virtually any large foreign molecule can act as an antigen, including those contained in bacteria, viruses, protozoa, helminths, foods, snake venoms, egg white, serum components, red blood cells, and other cells and tissues of various species, including humans. An antigen that induces an immune response—i.e., stimulates the lymphocytes to produce antibody or to attack the antigen directly—is called an immunogen.

On the surface of the antigens are regions, called antigenic determinants, that fit and bind to receptor molecules of complementary structure on the surface of the lymphocytes. The binding of the lymphocytes' receptors to the antigens' surface molecules stimulates the lymphocytes to multiply and to initiate an immune response—including the production of antibody, the activation of cytotoxic cells, or both—against the antigen. The amount of antibody formed in response to stimulation depends on the kind and amount of antigen involved, the route of entry to the body, and individual characteristics of the host.