Blood Groups — the ABO and Rh Systems

Not every person's blood is chemically identical, and the differences matter enormously in medicine, particularly for safe blood transfusion. The surface of the red blood cell can carry molecular markers called antigens (or agglutinogens), and the plasma correspondingly carries antibodies (or agglutinins) directed against whichever antigen that person's own red cells do not display — this reciprocal antigen/antibody arrangement is the basis of the ABO blood grouping system, first worked out by the Austrian scientist Karl Landsteiner.

Two antigens, conventionally labelled A and B, are recognised in this system, and every person's red blood cells carry one, both or neither of them, giving rise to four possible ABO groups. A person of blood group A carries antigen A on the red cell surface and, correspondingly, carries anti-B antibody in the plasma (since the immune system does not normally produce antibodies against an antigen present on the person's own cells). A person of blood group B carries antigen B and anti-A antibody. A person of blood group AB carries both antigens A and B on the red cell surface, and consequently carries neither anti-A nor anti-B antibody in the plasma. A person of blood group O carries neither antigen on the red cell surface, and consequently carries both anti-A and anti-B antibodies in the plasma.

This pattern directly determines who can safely donate blood to whom: transfusing blood whose red cells carry an antigen that the recipient's own plasma has an antibody against would cause those donor red cells to clump together (agglutinate) inside the recipient's vessels, a dangerous and potentially fatal reaction. Since group O red cells carry no A or B antigen at all, they will not be attacked by any recipient's anti-A or anti-B antibody, which is why group O is called the universal donor. Conversely, since group AB plasma carries neither anti-A nor anti-B antibody, a group AB person can safely receive red cells carrying either or both antigens, which is why group AB is called the universal recipient. In practice, exact group-matched transfusion is still always preferred in modern medicine, and the universal donor/recipient labels describe only the ABO antigens, not the full range of other blood group antigens now known to exist. …