Blood Typing

At the dawn of the 20th century, the Austrian physician Karl Landsteiner made the extraordinary discovery that human blood could be grouped into several different types. This discovery made possible the transfer of blood from one human to another—a medical breakthrough that has saved countless lives.
Prior to Landsteiner's pioneering work, there were few reports on the transfer of blood from one human to another. In 1668 Jean Baptiste Denis, the French physician to King Louis XIV, dared to transfuse a man with sheep's blood. The man eventually died and Denis was arrested for murder. Transfusions were quickly banned in France and England. Other attempted transfusions using human blood were frequently unsuccessful, and patients often died due to blood incompatibility.
In 1900 Landsteiner made the brilliant observation that human blood contains what he called isoagglutinins. These proteins are capable of agglutinating, or clotting, the red blood cells of blood samples containing isoagglutinins different from their own. He thus was able to divide blood into three types: A, B, and O. A rare fourth type, AB, was later discovered.
Landsteiner showed that the sera of two blood samples containing the same isoagglutinins would not clot the red cells of either blood. This discovery permitted the development of a system for safe blood transfusions. For this gift to humankind, Landsteiner received the Nobel Prize in physiology or medicine in 1930.
source: encarta encyclopedia