424 BIOLOGY OF PNEUMOCOCCUS 



for the detection or quantitative determination of agglutinins in 

 immune serum, the agglutination reaction finds a wide use in the 

 identification of members of the types within the species. 



The precipitin reaction, involving as it does the interaction of 

 hapten and immune body in solution instead of the reaction be- 

 tween formed microbic elements and specific serum and, moreover, 

 being independent of the participation of the immunological fac- 

 tors possessed by a living, immune animal, is susceptible to the 

 application of mathematically accurate quantitative measure- 

 ments. By means of the precipitin reaction, it is possible to deter- 

 mine with great exactness the quantity of hapten in a given solu- 

 tion or the amount of immune nitrogen — that is, the amount of 

 specific antibody in immune serum. Because of the fortunate, close 

 parallelism existing between the content of precipitin and protec- 

 tive antibody in any particular serum, a quantitative estimation 

 of the therapeutic value of antipneumococcic serum can be made 

 in the test tube instead of in experimental animals. 



The greater economy of means required in the reaction and its 

 greater accuracy when properly performed have led to far more 

 detailed studies of the phenomenon which, in turn, have supplied 

 many clues to the mechanism operating in immunity to Pneumo- 

 coccus. The chemist has thus been given the means of learning the 

 very chemical radicals in the precipitinogen and the globulin frac- 

 tion of immune serum which join to produce the precipitate and 

 presumably which participate in the other immunological manifes- 

 tations. The specificity of serological precipitation depends on the 

 interaction of the capsular polysaccharide and immune globulin; 

 and the basic process is the same as that in the phenomenon of 

 agglutination. 



The method of complement fixation offers no advantages over 

 other serological methods for the demonstration or measurement 

 of pneumococcal antigen and antibody. It has, however, revealed 

 and enabled one to study the striking difference in the nature of 

 specific antibodies in immune rabbit serum and immune horse serum. 



