antigen an·ti·gen (ān'tĭ-jən) n. Any of various substances, including toxins, bacteria, and the cells of transplanted organs, that when introduced into the body stimulate the production of antibodies. Also called allergen, immunogen.
(ān'tĭ-jən) A substance that stimulates the production of an antibody when introduced into the body. Antigens include toxins, bacteria, viruses, and other foreign substances. Compare antibody. See Note at blood type.