ANTIBODIES TO PNEUMOCOCCUS 373 



zonal irregularities. Smith 1298 later (1932) applied the principle 

 of optimal proportions to precipitin tests with Type I soluble 

 specific substance and homologous antibody and reported that the 

 method gave a true index of the protective power of the serum. 

 More recently Felton devised a test for the combining power of 

 antipneumococcic serum with specific polysaccharide. 



In the description by Francis, 472 already cited in the discussion 

 of agglutinins, it appeared that differences in the ratio of antigen 

 and antibody which affect the agglutination reaction apply also 

 to the precipitation reaction. When a solution of soluble specific 

 substance was added in excess to homologous immune serum, a 

 pro-zone was created in which precipitation was inhibited. As in 

 the agglutination reaction, Francis showed that it was the cap- 

 sular polysaccharide which constituted the precipitinogen. 



Quantitative relations in precipitation. In 1929, Heidelberger 

 and Kendall 616 published the first of a series of communications on 

 a quantitative study of the precipitin reaction as related to sol- 

 uble specific substance and homologous immune serum. After pre- 

 cipitating Type III antipneumococcic serum with Type III poly- 

 saccharide, the authors measured the amount of nitrogen in the 

 supernatant fluid of the precipitated mixture, and then from the 

 nitrogen content calculated the amount of protein remaining in 

 solution. The authors then tested the experimental data by the 

 law of mass action to determine whether the reaction showed 

 analogies to the behavior of simpler ionic reactions. Depending on 

 the relative amounts of the reactants, the specific precipitate ap- 

 peared to be a mixture of 'varying proportions of two compounds, 

 or a whole series of compounds, containing hapten and antibody 

 in varying proportions, whose limits could be expressed by the 

 following equilibriums, A and S being respectively equivalent 

 amounts of antibody and Type II soluble specific substance : 



(1) A + S^± AS (120:1 ratio) 



(2) AS + S ^± AS 2 (60: 1 ratio) 



