372 BIOLOGY OF PNEUMOCOCCUS 



stimulating the production of specific antibodies, was also anti- 

 genic in that, in a highly purified form, it reacted specifically with 

 homologous precipitin and completely removed precipitin from 

 immune serum. 



Balance between precipitinogen and precipitin.Morga.n (1923) 912 

 demonstrated that a proper balance between antigen and anti- 

 body was required to bring about the phenomenon of precipita- 

 tion. In order to obtain the maximal precipitate from any given 

 quantity of antipneumococcic serum, a definite or optimal amount 

 of precipitinogen or soluble specific substance was necessary. The 

 ratio between the quantity of immune serum and reacting sub- 

 stance was found to be practically constant. Therefore, when an 

 excess of precipitinogen was present, the resulting precipitate 

 was not only decreased in amount, as determined by inspection and 

 weight, but its appearance was altered. The importance of the 

 relative proportions of antigen and antibody was also demon- 

 strated by Dean and Webb (1926) 311 who, employing normal 

 horse serum and its homologous antiserum, devised a method by 

 which it was possible to determine quantitatively the amount of 

 antigen and antibody and therefore the optimal ratio between the 

 two components necessary for complete precipitation. 



Precipitin index. Sobotka and Friedlander (1928) 1299 deter- 

 mined that the sensitivity of the precipitin reaction with anti- 

 pneumococcic serum could be expressed by the product of the con- 

 centrations of the two reacting substances, antigen and antibody, 

 and defined the precipitin index (P.I.) as one-millionth of the re- 

 ciprocal value of the product. In investigating the zonal phenome- 

 non and its bearing on the absolute concentration and equivalent 

 weight of antibody, the authors found that the greater tendency 

 toward the exhibition of a post-zone in Type III precipitation 

 was connected with the lower acid equivalent of the homologous 

 capsular polysaccharide. The addition of normal serum as well as 

 an increase in pH promoted the post-zonal effect. By determining 

 the precipitin index, it was possible to recognize and to eliminate 



