This article waslast modified on 10 July 2017.
Substance (e.g., a toxin) or organism (e.g., an amoeba) that, when entering the body, causes the production of an antibody that reacts specifically with the antigen to neutralize, destroy, or weaken it. The presence of certain antigens is the criterion for typing in the ABO blood group system and is important in tissue cross-matching for transplants (e.g., the HLA antigen in kidney transplants).