The development of blood transfusion, a potentially life-saving technique, was hampered by a serious problem: many times the patient would suffer an often fatal "transfusion reaction," apparently to the donated blood itself. The cause of such reactions was unknown.

In the late 1800s, several researchers noted that when blood cells from one animal or person were mixed with cells from another, agglutination occurred--the cells stuck together in clumps. An Austrian physician named Karl Landsteiner showed what caused this clumping in 1900. Mixing different samples of human red blood cells and human sera (blood without the cells), Landsteiner observed that one mixture would clump, while another different mixture would not. He determined that the different reactions were caused by differing antigens in the red blood cells reacting to antibodies in the sera. In 1901, Landsteiner published a paper describing a method of categorizing human blood into three groups, or...