CHAPTER 2 

 Antibodies II 



Specificity and Chemical Structure of Antigen 



We are now in a position to investigate the effect on serological 

 specificity of known variations, large or small, in the chemical com- 

 position of cross-reactive antigens. Suppose, for example, we replace 

 the metanilic acid which we employed in making the azo chicken 

 serum CM with a different aromatic amine, which we may designate 

 as M'. Then if we test CM' with our antiserum against horse- 

 metanilic acid, a positive reaction will depend on whether M' is suf- 

 ficiently similar to M to combine with anti-M. If M' is w-amino- 

 benzoic acid or w-aminoarsenic acid, for example, a positive reac- 

 tion may be obtained with an antiserum to horse-metanilic acid, 

 although the amount of precipitate will be less. This shows that the 

 antibodies to metanilic acid, though best adapted to the homologous 

 hapten, can react also with other haptens having a dift'erent acid 

 group in the meta position. If we make the difference greater by em- 

 ploying a hapten with the acid group in the para position, or use a 

 hapten without any acid group, little if any cross-reaction is ob- 

 served (Fig. 2-1). We conclude that antibodies are able to dis- 

 tinguish structural isomers (molecules containing the same groups 

 but in different positions) but can also distinguish simple groups 

 (acid groups in this instance) which occupy the same position. 



It is a characteristic of antisera that the antibody molecules they 

 contain are not all alike ; they may differ in strength of reaction with 

 a given hapten and may differ in their specificity. This is easily shown 

 in the present case by allowing the anti-metanilic acid serum to react 



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