256 THE ANTIGEN-ANTIBODT REACTIONS 



dicate the instances in which precipitation occurs. The reaction is strongest with 

 the homologous antigen, containing the [J — SO3H group ; but precipitation 



occurs when the — SO3H group is shifted to the meta position f(^ ^ jjOr 



I 

 when it is replaced, in the ortho position, by — COOH/^Q coOh)' ^^ ^^^^ ^^^ 



SO3H 

 occur when the — SO3H group is shifted to the para position iC) j, or when 



the — SO3H group in the ortho position is replaced by a — COOH group in the meta 

 position (M ). The reaction is also abolished by introducing a methyl 



I 

 or chlorine group in the para position, even though the — SO3H group is retained 



in the ortho position. 



Another example, illustrating the importance of spatial configuration and the 

 immunological equivalence of different chemical groups so long as this spatial 

 configuration is maintained, is afforded by the studies of Erlenmeyer and Berger (1932). 

 They found that azo-proteins prepared from the compounds NH2<^ ) — ^0 — ( ), 

 NH2< )— NH — (^ and NH2<^— CHj— <^ behaved similarly in precipitation 

 reactions, though NH^^ ) — C — ( ) reacted differently. 



The studies of Landsteiner and van der Scheer (1932a, 19346) on peptides also 

 illustrate the effect of relative position on active groups in a complex molecule. 



They prepared synthetic azo-proteins from the dipeptides glycyl-glycine, leucyl-leucine, 

 glycyl-leucine and leucyl-glycine. It was found that the terminal amino-acid carrying the 

 carboxylic group had the greater influence on immunological specificity. Thus a glycyl- 

 leucine a,ntiserum reacted best with the corresponding antigen, less effectively with a 

 leucyl-leucine antigen and much less effectively with a glycyl-glycine or leucyl-glycine 

 antigen. The nature of the penultimate amino-acid also influenced the reaction, and 

 in later studies (1939) with pentapeptides made from various combinations of glycine and 

 leucine, it was found that the arrangement of all five members of the polypeptide chain 

 affected specificity. 



The influence of the whole substituted group, apart from that of active acidic or other 

 sub-groups, is well shown in the studies of Erlenmeyer and Berger (1932), Mutsaars and 

 Gregoire (1936), and Jacobs (1937), with compounds containing more than one benzene 

 ring. Here the nature of the whole compound was prominent in determining specificity. 



To obtain precipitation with these synthetic antigens and the corresponding 

 antisera it is in most cases necessary to employ the complete antigen, i.e. the 

 active group coupled to a suitable protein. 



The specificity of an antiserum, however, can be tested by adding to it the 

 active substance, alone or coupled to a substance simpler than protein. Combina- 

 tion of the group with the antibody is then tested by adding the complete antigen. 

 If precipitation is inhibited, we may conclude that the simpler fragment of antigen 

 has combined with the antibody, forming a soluble compound and blocking its 

 combining groups. 



