The first divisions of blood into groups were based on differences among the antigenic substances on the surfaces of the red cells of a given species of animal. These antigens are distinguished by means of antibodies in serum that combine with the red cells. Blood-group characters are inherited according to simple Mendelian laws, and the antigens, believed to be products of allelic, or closely linked, genes, are classified together in bloodgroup systems (1). Inherited variations in serum proteins (2) and enzyme activities in both serum and red cells (3) are other factors now known to differentiate bloods within a species, but the blood-group substances discussed in this article are those related to the groups defined by the red cell antigens.The first human blood-group system, the ABO system, was discovered by Landsteiner (4) as a result of his attempts to determine whether specific serological differences existed between individuals of the same species, and the importance of a knowledge of the ABO groups for the safe practice of blood transfusion was promptly recognized. The second blood-group system of outstanding clinical significance, the rhesus (Rh) system, was not discovered until 40 years later (5). In the intervening years two other systems, the MN and the P systems, were found from examination of serums 172 The first divisions of blood into groups were based on differences among the antigenic substances on the surfaces of the red cells of a given species of animal. These antigens are distinguished by means of antibodies in serum that combine with the red cells. Blood-group characters are inherited according to simple Mendelian laws, and the antigens, believed to be products of allelic, or closely linked, genes, are classified together in bloodgroup systems (1). Inherited variations in serum proteins (2) and enzyme activities in both serum and red cells (3) are other factors now known to differentiate bloods within a species, but the blood-group substances discussed in this article are those related to the groups defined by the red cell antigens.The first human blood-group system, the ABO system, was discovered by Landsteiner (4) as a result of his attempts to determine whether specific serological differences existed between individuals of the same species, and the importance of a knowledge of the ABO groups for the safe practice of blood transfusion was promptly recognized. The second blood-group system of outstanding clinical significance, the rhesus (Rh) system, was not discovered until 40 years later (5). In the intervening years two other systems, the MN and the P systems, were found from examination of serums 172 from rabbits injected with different samples of human red cells (6). The antigens in the MN and P systems are of little clinical importance because the corresponding antibodies occur infrequently in man and seldom produce untoward transfusion reactions. At the time of their discovery, however, the inheritance of very few normal human characters was established, and the M and N blood-group ...
