BLOOD GROUPS, HETEROGENETIC ANTIGENS 483 



and hemolysins, which may be produced by injection of red corpuscles of one 

 species into a strange species. These immune agglutinins and hemolysins also 

 possess a species-specific character; the presence of such species-specific sub- 

 stances may obscure the existence of the group differentials, and in the case 

 of immune agglutinins which are directed against human erythrocytes it may 

 be necessary first to absorb the species-specific heteroagglutinins by human 

 corpuscles of Group I, which possess neither the A nor the B group differen- 

 tials, if a test is to be made of the presence of group agglutinins in this serum. 



There exist, then, marked differences between the individuality differentials 

 demonstrable by means of transplantation and the differentials of the four 

 primary blood groups. It seems that it was the proof that very fine differences 

 between individual constitutions can be established by means of transplanta- 

 tion which led immunologists to seek likewise for methods making possible 

 finer differentiations between individuals by means of blood grouping. Ac- 

 cordingly, in recent years, by the use of immune agglutinins in addition to 

 the natural blood-group agglutinins, Landsteiner succeeded in adding new 

 groups to the four primary blood groups. Thus within the Group A, Land- 

 steiner distinguished between two subgroups, A x and A 2 ; these differ in the 

 way they unite with two subagglutinins; alpha! and alpha 2 . In a somewhat 

 related way Thomsen distinguished between the original Group A and the 

 subgroup of the latter, A x . A x corpuscles are less intensely agglutinated by 

 antisera than are the typical A corpuscles. Thomsen thus adds to the differen- 

 tials A, B and A+B, a fourth one, A x . To these subgroups correspond sub- 

 groups among the agglutinins of the normal human sera. 



Also, the B differential in human erythrocytes has recently been further 

 differentiated into a B x component, which so far seems to be peculiar to human 

 cells, and into B 2 and B 3 , which occur, besides, in the blood corpuscles of 

 certain animals, such as the rabbit. Correspondingly, anti-B of human sera 

 may contain a mixture of anti-B x and anti-B 2 ; however, not all human sera 

 contain the anti-B! component. 



In addition, Landsteiner and his associates established three further sub- 

 groups carrying the agglutinogens M, N and P, respectively. No preformed 

 agglutinins corresponding to the agglutinins M and N exist in normal human 

 sera, but they can be produced through immunization of rabbits with these 

 antigens. Moreover, in contradistinction to the primary blood-group antigens, 

 M and N have not been found in cells other than the erythrocytes. These 

 additional agglutinogens occur probably in all of the four primary blood 

 groups and they, together with the ordinary blood groups and subgroups A x 

 and A 2 differentials, make possible the differentiation between thirty-six 

 classes of individuals. In this way the ability to differentiate between different 

 individuals is much increased. 



More recently, Schiff, through immunization of a sheep with the blood of a 

 person belonging to Group O and possessing M, N and P differentials, estab- 

 lished the existence of still another differential in human corpuscles, which 

 he designates as H, and which may be present in any of the four primary 

 human blood groups ; it seems to be transmitted to the offspring by means of 



