90 



INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



Some of this information may eventually see practical application, 

 for Kabat suggests that the introduction of a number of melibiosyl 

 residues into a polysaccharide would endow it with substantial blood 

 group B activity. 



How many other sugars are present in the active side chains of 

 the A and B molecules is not known, but on the basis of his determi- 

 nations of the size of the reactive group of dextran (p. 48) Kabat 

 believes that the total is of the order of six. This would mean that 

 in the light of our present knowledge the specific portions of the 

 A and B molecules would resemble the structures shown in Fig. 

 7-6, where x stands for a number of the order of four. 



H NHCOCH3 H OH 



A Substance 



X = approx. 4 

 Fig. 7-6. Suggested structures of reactive groups of blood group A 

 substances. 



and B 



Additional and independent evidence for the part played by 

 L-fucose, A^-acetylgalactosamine, and D-galactose in H, A, and B 

 specificity, respectively, was obtained by Watkins and Morgan (1955) 

 from the results of enzyme inhibition by these sugars. It is known 

 that an enzyme can be inhibited by an excess of one of the products 

 of its action on its substrate. An enzyme preparation was available 

 from Trichomonas foehts which destroyed the substrates consisting 



