Any substance that the body regards as foreign and that therefore elicits an immune response, particularly the formation of specific antibodies capable of binding to it. Antibodies recognize and bind to specific structures on the surface of the antigen called epitopes. Antigens may be formed in, or introduced into, the body. They are usually proteins. Histocompatibility antigens are associated with the tissues and are involved in the rejection of tissue or organ grafts (see histocompatibility); an example is the group of antigens encoded by the HLA system. A graft will be rejected if the recipient’s body regards such antigens on the donor’s tissues as foreign. See also antibody; b-cell receptor; t-cell receptor. Compare hapten.