Why did so many soldiers wounded during the civil war die after given a battlefield transfusion? To answer this question, the discovery of different "types" of blood amongst humans was investigated.
Red blood cells may contain on their membranes, specific protein antigens. Two specific antigens were discovered, Antigen A and Antigen B. Four specific blood types were found, depending on whether the blood cell contained antigen A or Antigen B or both or neither. The types were classed as type A, type B, type AB or type O. Also, the plasma of human blood contained specific antibodies related to those antigens. They were antibody A (anti A) and antibody B 9 anti B). Type A blood contained Anti B antibodies, type B contained anti A antibodies. Type AB contained neither anti A or anti B, while type O contained both anti A and anti B. The agglutination caused by transfusing the wrong type of blood was discussed as being caused by the donors blood clotting and not the recipients.