206 



THE ANTIGEN-ANTIBODY REACTIONS 



to the amount and composition of the precipitate. The antibody excess zone is 

 to the left, the equivalence zone or point is indicated by the arrows, and the antigen 

 excess zone is to the right. As the amount of antigen added is increased, the amount 

 of antibody in the precipitate rises to a maximum. The maximum is not, as 

 might be expected, attained in the equivalence zone, but increases in the antigen 



0-02 0-04 06 008 010 012 0-14 0-16 0-18 020 060 10 40 80 

 Mgm. Antigen Added. 

 Fig. 32. — Amounts of antibody nitrogen and antigen in the jirecijjitate formed from 1 ml. of 



antibody solution, on the addition of the amounts of antigen shown as abscissae. 

 Continuous lines antibody nitrogen ; broken lines antigen ; Type III pneumoeoceus polysac- 

 charide (SSS III) in mgm. of polysaccharide. 



X SS8 III and homologous antibody. 

 O Azo-protein and homologous antibody. 

 /\ Egg albumin and homologous antibody. 

 The arrows sliow the points at which neither antigen nor antibody was detectable 

 in the supernatant fluids. The figures against points on the curves show the ratio of 

 antibody to antigen in the precipitate at these points. 



The scale of the abscissae is changed twice in order to bring the whole curve into the figure. 

 From the figures of Heidelberger and Kendall (1935). 



(Reproduced from Marrack (1938) by kind permission of the Author and H.M. Stationary 

 Office.) 



excess zone. According to tests of the supernatant fluid, all the available antibody 

 was removed from mixtures in the equivalence zone, so that the antibody repre- 

 sented by the extra weight of precipitate in the antigen excess zone must be 

 incapable of forming a precipitate by itself, though capable of entering into com- 

 bination with antigen and precipitating antibody. The existence of such antibody 



