Substances, usually harmful, that when entering the body stimulate the immune system to produce antibodies. Invaders may be polio virus, typhoid bacillus, or chain-like streptococcal bacteria – the usual pus-forming type found in infected wounds, or toxins they release into the blood stream.
a group of antigens that may or may not be present on the surface of red blood cells, forming the basis of a *blood group. This group is important in blood transfusion reactions. [Mrs Kell (20th century), patient in whom they were first demonstrated]... kell antigens