1. 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 neutralise, destroy or weaken it.
2. The presence of certain antigens on blood cells is the basis for blood typing for transfusions. Antigens that are present on tissue allow for donor-recipient matching in transplant medicine.