The blood of each person seemed to carry in it the secrets of individuality. If each persons blood were as individual as this, transfusion would indeed be complex and would deserve to rank as the most refined branch of medicine. However, this early view of the subtlety of transfusion was eclipsed at the beginning of the century by the discovery that the blood of all human beings could be divided into four groups. It seemed that, provided blood of the same group was transfused, one persons blood was indistinguishable from anothers. Indeed, it came to be believed that people who belonged to the...