Antigen (Gr. Anti — Against, Gennan — Generate)

An antigen is a foreign substance, usually a protein or a polysaccharide-protein complex, which, when introduced into the body, causes the formation of specific antibodies. Antigens can be of exogenous origin (eg, bacterial toxins) or endogenous (eg, tissue antigens). The interaction of antigen with antibody is the basis of the immune response. Recognition of an antigen by the immune system initiates a cascade of reactions aimed at neutralizing it and removing it from the body. Understanding the nature and properties of antigens is extremely important for studying the mechanisms of immunity and developing vaccines.