448 BACTERIOLOGICAL CHEMISTRY 



8liown thai the optiininn [)i'()poi*tioii of a-ntigen and 

 antibody, although giving the most rapid precipitation, 

 did not, as a rule, give the greatest amount of precipitate, 

 but that this usually occurred in the region of antigen 

 excess. 



In the presence of a considerable excess of either 

 antigen or antibody no precipitation occurs, and a pre- 

 cipitate already formed may dissolve on adding an excess 

 of either reagent. This is the so-called zone phenomenon ; 

 the precipitation only occurs over a limited range of 

 antigen-antibody proportions. The variation of antigen- 

 antibody proportions may be brought about in one of 

 two ways : (a) the amount of antibody may be kept 

 constant and the amount of antigen varied, as is the 

 usual practice in precipitin reactions ; or (5) the amount 

 of antigen may be kept constant and the amount of 

 antibody varied, as is the normal procedure in carrying out 

 agglutination reactions and the toxin-antitoxin reactions. 

 It has been found that the optimal proportions as deter- 

 mined by these two methods are not the same, but vary 

 to an extent depending on the particular system being 

 studied. For example, in the case of the Ramon floccula- 

 tion reactions of diphtheria toxin and antitoxin, the 

 optimum proportion is 1 : 8 as determined by the constant 

 antibody method, and 1 : 64 as determined by the con- 

 stant antigen method. 



Since definite, chemically pure antigens have become 

 available a considerable amount of accurate quantitative 

 work on the composition of the precipitates has been 

 possible. Dean and Webb estimated that horse serum 

 as the antigen formed about 12 per cent, of the precipitate 

 at the optimum proportion, and that it fell to about 

 6 per cent, in the region of antibody excess. Heidel- 

 berger and Kendall found that Type III pneumococcus 

 polysaccharide (which contains no nitrogen) formed 



