/an"ti jeuhn, -jen'/, n.1. Immunol. any substance that can stimulate the production of antibodies and combine specifically with them.2. Pharm. any commercial substance that, when injected or absorbed into animal tissues, stimulates the production of antibodies.3. antigens of a particular type collectively.[1905-10; ANTI(BODY) + -GEN]
* * *Foreign substance in the body that induces an immune response.The antigen stimulates lymphocytes to produce antibodies or to attack the antigen directly (see antibody;immunity). Virtually any large foreign molecule can act as an antigen, including those of bacteria, viruses, parasites, foods, venoms, blood components, and cells and tissues of various species, including other humans. Sites on the antigen's surface fit and bind to receptor molecules on the lymphocytes' surface, stimulating the latter to multiply and initiate an immune response that neutralizes or destroys the antigen.
* * *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.
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