76 INTRODUCTION TO lAIMUNOCHEMICAL SPECIFICITY 



whereas the Dohchos lectin is not specilic fur any of these, hnt 

 detects a previously undescrihed heterogenetic factor present in 

 the erythrocytes of sheep, goat, horse, dog, and pig (Bird, 1959). 



The anti-A of lima beans is somewhat less specific ; its affinity 

 for A2 is higher, and, when concentrated, it weakly agglutinates 

 B cells also. The anti-H lectins are still less specific, for, when 

 sufficiently concentrated, they may agglutinate Ai and B cells. This, 

 however, might be because human erythrocytes of these groups con- 

 tain some H antigen. 



In addition to these relatively specific lectins, others are known 

 which seem to react with more than one receptor on the red cell, 

 such as A and B, or B and H. Finally, we come to those which 

 agglutinate human cells of all groups. Even these, however, may have 

 their own specificity, as I shall mention later. 



As we shall see in the next chapter, human isoagglutinins react 

 not only with purified A and B substances but also with certain frag- 

 ments into which these antigens can be split, as by hydrolysis. In 

 the case of fragments too small to precipitate, specific activity has to 

 be demonstrated by the inhibition technique. In view of what we 

 know about antigenic determinants, we should expect a limit to this 

 fragmentation process ; i.e., if the A blood group antigen, for example, 

 is split into portions that are too small, the fragments will no longer 

 show A specificity or, in other words, will no longer inhibit the re- 

 action of anti-A agglutinin with A antigen. It was found the smallest 

 fragment of the A antigen which specifically inhibited the reaction 

 of human anti-A antibody with the A antigen is a disaccharide con- 

 taining A^-acetylgalactosamine as a terminal group. (The structure 

 of this disaccharide will be discussed in Chapter 7.) Strikingly 

 enough, anti-A agglutinins from plants were inhibited not only by 

 this disaccharide but by A^-acetylgalactosamine itself (Morgan and 

 Watkins, 1959). From this observation we could draw either of 

 two opposed conclusions. We could say that plant anti-A reagents 

 are less specific than human anti-A, since they are inhibited by a 

 simpler substance, or we could say they are more specific, since 

 they cross-react less with other portions of the A antigen. 



In the case of anti-H agglutinins the plant reagents have not proven 

 to be any less specific than those of animal origin. The anti-H of eel 

 serum is inhibited by L-fucose, and L-fucose inhibits the anti-H of 



