BLOOD GROUPS 57 



This theory has been supported l)y Furiihata (1927) and tlie Wurni- 

 sers ( Fihtti-Wurmser et al., 1954). Whether this theory seems 

 plausible depends partly on which theory of antibody formation we 

 happen to believe. 



Another theory suggests that anti-xA. and anti-B are immune anti- 

 bodies, as most other agglutinins are, being formed in response to 

 antigens, in food, in bacteria, and in animal parasites, which are 

 chemically similar to the A and B antigens of man. It is known that 

 a number of such related antigens exist ; in Chapter 2 the cross- 

 reaction of blood group A antigen and type 14 pneumococcus was 

 mentioned. 



If the second theory of isoagglutinin formation is correct, one 

 wonders why natural isoagglutinins for other human blood antigens, 

 such as M, N, and Rh, are so seldom encountered. One possible rea- 

 son is the lower antigenicity of these agglutinogens ; some evidence 

 for this exists. Another reason might be that the human agglutino- 

 gens M, N, and Rh are more unusual in their structure than A and B, 

 so that closely related antigens in lower organisms and in food, serv- 

 ing as stimuli for the formation of anti-M, anti-N, and anti-Rh anti- 

 bodies in man, are only rarely encountered. Later on I shall mention 

 some recent evidence in support of this speculation. 



MNS Blood Groups 



Although unknown for so long, the existence of the ABO blood 

 groups was relatively easy to demonstrate because of the normal 

 presence of the isoagglutinins anti-A and anti-B. However, over a 

 quarter of a century went by before any other blood group system 

 was discovered in man. In 1927, Landsteiner and Levine demonstrated 

 the existence of M and N antigens (also P) by the use of absorbed 

 sera of rabbits injected with human erythrocytes. Because anti-M 

 and anti-N isoagglutinins rarely occur and because patients are not 

 readily immunized to these antigens, they have little importance for 

 transfusion ; their applications have been mainly to legal medicine 

 and to anthropology. 



A number of variants of the M and N antigens and, in addition, 

 other antigens associated with the M and N factors in inheritance 

 have been discovered. Two of these, S and s (to be distinguished 



