The major human blood group system. A person's ABO type depends upon the presence of absence of two genes --the A and B genes. These genes are encoded on chromosome 9 (in band 9q34.1). They determine part of the configuration of the red blood cell surface.

A person can be A, B, AB, or O. If a person has two A genes, their red blood cells are type A. If a person has two B genes, their red cells are type B. If the person has one A and one B gene, their red cells are type AB. If the person has neither the A nor B gene, they are type O.

The situation with antibodies in blood plasma is just the opposite. Someone with type A red cells has anti-B antibodies (antibodies directed against type B red cells) in their blood plasma. Someone with type B red cells has anti-A antibodies in plasma. Someone who is type O has both anti-A and anti-B antibodies in plasma. And someone who is type AB has neither anti-A nor anti-B antibodies in plasma.

It is most important to determine the ABO status of both donor and recipient in transplants and transfusions by typing and cross-matching. ABO incompatibility in such procedures can be a disaster.

The first recorded blood transfusion may have taken place in 1492 when Pope Innocent VIII, laying in a coma, was given the blood of 3 young men. Blood typing and crossmatching was not done. The pope died, as did the 3 donors.

In 1901 a Viennese pathologist named Karl Landsteiner (1868-1943) published an article entitled "On Agglutination Phenomena of Normal Human Blood," in which he observed that, when blood was transfused from one human to another, the body often clumped the transfused blood cells and rejected the transfusion, sometimes going in shock. In 1909 Landsteiner classified red blood cells into types A, B, AB and O and showed that the body rejects transfusions of a different blood type. After moving to the Rockefeller Institute in New York, Landsteiner received the Nobel Prize in 1930 for his pioneering research in immunology and blood grouping.