PLANT AGGLUTININS (LECTINS) II 75 



lectin more specific by treating it with B cells to remove the anti-B, 

 we find we can do so only by absorbing an amount of the protein 

 sufficient to reduce the lectin concentration to such a point that it 

 will no longer agglutinate 1j cells (Table 6-1). This is seen from 

 the fact that, while the absorption was successful in lowering the 

 titer of the lectin against B cells about two stages, it also lowered 

 the titer against Ai cells two stages (Boyd and Shapleigh, 1954d). 



Another indication of the homogeneity of lectins is the observation 

 of Morgan and Watkins (1953) : the agglutinin of SopJiora japonica 

 can be absorbed by either Ai or B cells, but the agglutinin, on being 

 eluted from either type of cell, still agglutinates A and B cells as 

 before, showing that no separation into molecules of different speci- 

 ficities has been effected. 



This homogeneity of the lectins possibly sheds light on what their 

 function may be in the plant, and also, because of its contrast with 

 antibodies, has a bearing on theories of antibody formation. 



Specificity of Plant Agglutinins 



We have already seen that there are degrees of specificity. The 

 specificity of an antibody may be described as lozv if it reacts with 

 a large number of antigens, especially if they are closely related 

 chemically. Specificity is said to be sharp if an antibody reacts only 

 with a small group of chemically closely related antigens. An antibody 

 may be said to have absolute specificity if it reacts with only one 

 antigen. 



When blood group-specific plant agglutinins were first discovered, 

 it was natural to suppose, since their reaction with the blood group 

 antigens was thought to be merely a chemical accident, that their 

 specificity was less sharp than that of the normal isoagglutinins. The 

 contrary has proved to be true, at least in some cases. 



The specificity of some lectins is very sharp. The anti-A of human 

 group B plasma, for example, reacts nearly as well with Ao as with 

 Ai cells. The lectin of Doliclws biflorus, on the other hand, has a 

 much greater affinity for Ai than for A2 (over 500 times as great), 

 so that it is virtually specific for Ai. Human anti-A reacts also with 

 the Forssman antigen, the J substance present in the blood of some 

 cattle, the R antigen present in some sheep, and hog A substance. 



