406 BACTERIOLOGICAL CHEMISTRY 



were coupled with siilphanilic acid, NH2<^ \sO3H, 



instead of with atoxyl, again the original protein specificity 

 disappeared, nor would sulphanilic acid-azo -proteins 

 react with antisera to atoxyl-azo -proteins and vice versa. 



This specificity can be demonstrated in another way. 

 The formation of the precipitate by the reaction of the 

 antiserum with the atoxyl-azo -protein antigen, for 

 instance, can be prevented by the previous addition of 

 simple atoxyl derivatives to the antiserum. Atoxyl 

 diazotised and coupled with tyrosine, or even atoxyl 

 itself, can act in this way. These simple derivatives are 

 not themselves antigenic nor do they give any visible 

 reaction with the antiserum. They are named simple 

 haptens. The inhibition of precipitin reactions by haptens 

 is also specific ; atoxyl haptens inhibit the reactions of 

 atoxyl antisera but not those of sulphanilic acid antisera, 

 whilst the sulphanilic acid haptens inhibit only reactions 

 between sulphanilic acid-azo -proteins and the corres- 

 ponding antisera and not those between any other antigens 

 and antibodies. 



The groups, like atoxyl or sulphanilic acid, which 

 modify the specificity of antigens in these ways are called 

 determinant groups. Landsteiner studied a large number 

 of aromatic amino -acids from this point of view and found 

 that the specificity which they introduced depended 

 partly on the substituent and partly on its position in 

 the ring. Thus antisera to o-amino -benzene -sulphonic 

 acid-azo -proteins gave reactions with both the 0- and 

 m-derivatives but not with the ^-derivative, showing the 

 effect of position. The same antisera also reacted with 



o-amino -benzoic acid, \ / ^ , as hapten, or, when 



COOH 



coupled with protein, as antigen, ])ut they showed no 

 reactions with the m- and ^-amino -benzoic acids. This 

 illustrates the com])ined effect of the substituent and its 



