SPECIFIC INHIBITION 



257 







COOH 



When this procedure is adopted, it is found that relatively simple substances 

 may function as incomplete, or partial antigens, in the sense that they combine 

 specifically with corresponding antibodies. Thus, diazotized atoxyl coupled with 

 tyrosine, instead of with an intact pfotein molecule, does not form a precipitate 

 when mixed with the corresponding antiserum, but it inhibits the precipitation 

 that would normally occur when this antiserum is mixed with an atoxyl-azo- 

 protein. A similar inhibition may be demonstrated with atoxyl itself in higher 

 concentrations, or even with arsenic acid when this is added in sufficient amount 

 (Landsteiner 1920). 



When the inhibition reaction is used to determine the immunological behaviour 

 of non-identical but structurally related organic compounds, it is found that the 

 specificity is less strict than that displayed in cross-precipitation tests. Fig. 43 

 illustrates the findings in relation to o-amino- benzoic acid (see Landsteiner and van 

 der Scheer 1931, Marrack 

 1938). The antibody pre- 

 pared against an azo-protein 

 containing this grouping, indi- 

 cated at the top of the figure, 

 gives specific precipitation 

 with another azo-protein, 

 containing the same active 

 grouping but a different pro- 

 tein-component, indicated at 

 the bottom of the figure. 

 Inhibition by various other 

 groupings is indicated by 

 the horizontal broken arrows. 

 The reaction is inhibited 

 not only by benzoic acid, or 

 benzoic acid substituted either 

 in the ortho or meta position 

 by the methyl group, but by 

 compounds showing consider- 

 able structural differences, 

 such as thiophene carboxylic 

 acid and naphthoic acid. 



The specific combination 

 of certain azo-dyes with 

 the corresponding antibodies, 

 without the formation of 

 precipitate, has also been 



NH< 







S 



cc 



CO-- 



»COOH 



S 

 COOR 



— -0 



— ^> 



— -0 



COOH. 



COOH. 



COOH 



a 



demonstrated by Marrack 

 and Smith (1932), using an 

 ingenious colorimetric tech- 

 nique. 

 We 



0. 



NHt 



I COOH 



NH2 



Fig. 43. — Inhibition. 



have already noted 

 that the partial antigens that inhibit precipitation do not generally themselves 

 yield a precipitate in vitro. Neither will they stimulate the production of pre- 

 cipitins in vivo. In the definition of an antigen that was given at the beginning 



P.B. K 



