n. any substance that is recognized by the immune system as being foreign and therefore dangerous. The body then produces antibodies that render it harmless. The antigen may be a virus, a bacterium, a toxin, or a tissue (e.g., blood) of another individual with different genetic characteristics following a transfusion. See also allergen.
ANTIGEN: "The antigen known as blood was recognized as foreign by the individual's body that had received the transfusion."