452 BACTERIOLOGICAL CHEMISTRY 



Similarly these new compounds can take part in a third 

 stage : — 



AS.A + A ^=^ AS.A.A. 

 AS.AS+A ^=^ AS. AS. A. 

 AS.A+AS.A. ?=^ AS.A.AS.A. 

 AS.A+AS.AS ^=-i AS.A.AS.AS 

 AS.AS+AS.AS ^=^ AS.AS.AS.AS. 

 This process is supposed to continue on similar lines 

 until insoluble aggregates are built up and precipitation 

 occurs. When antibody and hapten are mixed in equiva- 

 lent quantities the compound AS is believed to polymerise, 

 by a similar mechanism, to give (AS)n, which has the 

 composition of the precipitate at the equivalence point. 

 In the region of hapten excess analogous compounds are 

 formed until the inhibition zone is reached. The latter 

 only occurs when there is a considerable excess of hapten, 

 when all the specific groups of the antibody tend to react 

 with S rather than with AS or similar complexes, so that 

 no aggregation to form an insoluble precipitate takes 

 place. A similar explanation accounts for the non- 

 precipitation with simple haptens which contain only 

 one or two reactive groups, leading to the formation of 

 soluble compounds of the type AH^, which show no 

 tendency to aggregation. If, as in complex haptens like 

 the azo-dyes studied by Landsteiner, several reactive 

 groups are present, compounds of the type AH .AH ^re 

 formed, and aggregation followed by precipitation can 

 occur. 



Applying the Law of Mass Action to the above 

 reactions, Heidelberger and Kendall deduced that the 

 equation 



R2S2 



Mg. of antibody nitrogen precipitated = 2RS . (1) 



held in the region of antibody excess, where R is the ratio 

 of antibody to antigen at the equivalence point, S is the 

 amount of polysaccharide added and A is the amount 

 of antibody-nitrogen precipitated at the equivalence 



