Dealing with serious blood loss in an injured person has long been a difficult challenge for medical scientists. One solution has been to treat the patient with blood transfused from another person or an animal. In some instances, this approach worked and the patient recovered. In other cases in the past, however, the blood transfusion actually worsened the patient's condition and hastened death. Until the early 1900s, no one could predict which type of reaction would occur as the result of any one blood transfusion. As a result, most European nations had outlawed the practice of blood transfusion by the beginning of the nineteenth century.

An explanation for the phenomenon of blood rejection was developed around 1900 by Austrian-American physician Karl Landsteiner. Landsteiner found that human blood serum (the cell-free portion of blood) could be divided into four categories depending on its ability to cause clotting of...