492 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



antigen ; they absorb only traces of the human anti-A substance preformed in 

 human serum. This indicates that the Forssman differential and the human 

 A differential are not identical. 



In anti-human A rabbit immune serum which, as stated, contains also 

 Forssman antibodies, it is possible to remove the latter by absorption with 

 sheep corpuscles which contain the Forssman differential but do not contain 

 the human A differential; at the same time the ability of the immune serum 

 to react with alcohol extract from human corpuscles A is also diminished 

 by the removal of the Forssman antibodies. Forssman antibodies and anti- 

 bodies against human A differentials are perhaps in some loose manner linked, 

 and a certain constituent of the Forssman differential may occur in human 

 corpuscles belonging to group A. However, it may also be that the common 

 reactions of group A and Forssman differentials depend merely upon a 

 similarity in their chemical structure. 



In the case of the organismal differentials we have seen that the reactions 

 against strange differentials are not yet fully formed in very young organisms. 

 The development of the human blood-group differentials seems to set in at 

 about six or seven months of embryonal life and to be completed at the time of 

 birth. But it has been maintained that the full development of the A differen- 

 tial occurs only at the age of fifteen to twenty years. 



As to the agglutinins A and B which circulate in the blood serum, these 

 originate within the last two months of pregnancy; they develop therefore 

 later than the differentials present in the cells. In some cases they seem to be 

 lacking at the time of birth and to be formed only in the first few months of 

 extrauterine life. According to Thomsen, they reach their full development 

 only in children between five and ten years of age, and a decrease in the 

 quantity of these agglutinins may occur in old age; but this age involution 

 may take place fairly early and may therefore be found even in relatively 

 young individuals (Schiff and Mendlowitsch). 



If agglutinins are found in the blood of the newborn child, they may have 

 been derived from the mother, having reached the foetus by way of the 

 placenta. But in case mother and child belong to different blood groups, no 

 pathological effects seem to result from the combination of agglutinins in the 

 blood serum and the agglutinogens in the erythrocytes, which, theoretically, 

 should be expected to act on each other. It is assumed that mechanisms exist 

 which, as a rule, prevent the passing through the placenta of maternal ag- 

 glutinins capable of agglutinating the erythrocytes of the child. Von Oettingen 

 and Witebsky believed that the occurrence of the blood-group differentials 

 could not be demonstrated in the embryonal part of the placenta, although 

 they are found in the maternal decidua. According to Kritschewsky, it is the 

 Forssman differential which is present in the decidua and not the blood-group 

 differential, while the embryonal placenta is free of the latter. However, 

 Levine has found that the Rh antigen may pass in the uterus r from the child 

 to the mother. If the latter does not possess this antigen, antibodies may be 

 produced against it, which then pass in the opposite direction from the 

 mother to the child and here may cause erythroblastosis foetalis. 



