A substance which, when introduced into the body, evokes the production of one or more antibodies, by the immune system. The binding of antigens with antibodies occurs in a "lock and key" fashion.

Antigens are associated with: bacteria, pollen, foreign blood cells, toxins, viruses, and the cells of transplanted organs.

Red Blood Cell (RBC) antigens are attached to components in the membranes of the cells. (ABO blood group are sugars; Rh group are polysaccharides.)

Blood grouping is determined by the antigens (aka surface markers) found on RBCs. If a person has, for example, type A blood group, their immune system makes antibodies only for type B surface antigens. These antibodies do not recognise (key doesn't fit lock) type A antigens, so they ignore them. The type A person can receive blood from donors of types A or O, and can donate to recipients with types A or AB. The parallel situation occurs in people with type B blood.

People with type AB blood do not make antibodies for either type A or type B antigens, which means that they may receive blood from donors of any type - they are universal recipients.

Type O subjects do not have type A or type B antigens, so make antibodies for both type A and type B antigens. Consequently, these people may only receive blood from other type O donors, and are universal donors.