INDIVIDUALITY DIFFERENTIALS AND BLOOD GROUPS 153 



slight differences may lead to weak and often long delayed reactions. Especial- 

 ly significant in this connection are transplantations within closely inbred 

 strains; here the great difficulty in making the genetic constitution, which 

 determines the nature of the individuality differentials, identical, is clearly 

 demonstrated ; it is also shown that the reactions against the latter are parallel 

 to the differences in the genetic constitutions of host and donor and that these 

 differences show all kinds of gradations. Taking all of these facts together, 

 the only conclusion possible seems to be that many genes take part in the de- 

 termination of the nature of the individuality differentials. 



It seems therefore most probable that a further anlysis of the antigens pres- 

 ent in erythrocytes will show that they are either identical with or are a part 

 of the factors which call forth the individuality differential reactions against 

 all kinds of strange tissues. The peculiar character of the erythrocytes makes 

 it possible to obtain for the analysis of the constitution of these cellular ele- 

 ments, special hemolytic and agglutination reactions, which cannot be applied 

 generally in the study of the reactions against strange individuality differen- 

 tials ; but even in the case of the erythrocytes, as a rule it is not possible to dem- 

 onstrate the existence in the serum of the host of preformed hemolysins or ag- 

 glutinins for the red blood cells of the donor. Other characteristics which we 

 have discussed repeatedly make it possible to distinguish the individuality 

 differentials of cells and tissues in general, and there is, therefore, no reason 

 why the individuality differentials, which were established by entirely differ- 

 ent methods and which are common to all tissues, should be subordinated to 

 the factors which determine the agglutination and hemolysis of erythrocytes, 

 which latter represent very specialized modes of reaction between particular 

 kinds of cells and particular constituents of the blood serum. 



These conclusions as to the relations between the antigenic constitution of 

 the erythrocytes and the individuality differential do not necessarily apply if 

 instead of the numerous antigens of the erythrocytes, we consider merely the 

 four primary blood groups. By means of these it is possible to distinguish be- 

 tween certain individuals, and it is the identity or lack of identity of the blood 

 groups to which two individuals belong that determines the compatibility of 

 their blood in transfusions ; as stated the blood shows some analogies to 

 tissues ; therefore the compatibility of the blood might be taken as an indication 

 of the compatibility of the tissues comprising an individual. In pursuing this 

 trend of thought, several investigators went still further and considered the 

 blood group characters of an individual as the most significant features of his 

 constitution. 



The experimental data on which the evaluation of the correctness of this 

 interpretation has been based were obtained in a comparative study of the 

 results of skin grafting, in cases in which donor and host of the graft belonged 

 to the same or to different blood groups. The number of skin graftings in 

 man, in which the blood group relations between donors and hosts have been 

 considered, is great, but the conclusions arrived at by various investigators 

 differ very much. There are those who believe that the success of homoiog- 

 enous skin transplantations is determined by the blood group relations be- 



