PRACTICAL APPLICATIONS 373 



a dog against 'this amboceptor for his own cells by starting with very 

 small doses and gradually increasing these until it is found that the 

 animal tolerates amounts that would be fatal to non-immunized animals. 

 If a portion of this serum is now added to the specific hemolytic serum, 

 it will be found that the power of the latter is inhibited. Although this 

 action may likewise be due to anticomplement, it is probable that an 

 anti-amboceptor against the cytophile group of the amboceptor is also 

 formed, which prevents the amboceptor from uniting with the red blood- 

 cells, although conclusive experimental evidence of this has not been 

 adduced. 



PRACTICAL APPLICATIONS 



There is probably no other group of antibodies that possesses greater 

 diagnostic value than do the hemolysins, a fact that was demonstrated 

 in the practical application of the Bordet-Gengou phenomenon of com- 

 plement fixation in the diagnosis of syphilis and other infections. By 

 adding sensitized corpuscles i. e., corpuscles with their homologous am- 

 boceptors to a fluid, the presence or absence of complement may be 

 determined. If complement is present, hemolysis will occur and be 

 complete or partial, depending upon the amount of complement avail- 

 able; if hemolysis does not occur, it may be concluded that free com- 

 plement is absent. This is the basis of the complement-fixation diag- 

 nosis of syphilis, gonorrhea, glanders, differentiation of proteins, etc. 

 When a proper amount of complement is added to a mixture of antigen 

 and its immune serum (containing amboceptors), it is bound to these 

 amboceptors, so that when corpuscles and hemolytic amboceptor are 

 subsequently added, hemolysis does not occur, since there is no available 

 complement, it having been "fixed" by the first amboceptors. If, how- 

 ever, amboceptors for the antigen in the first instance are not present, 

 as where a normal serum is used, the complement remains free and 

 acts with the hemolytic amboceptor to produce hemolysis of the test 

 corpuscles. In this manner the hemolysins and their corresponding 

 corpuscles are employed as indicators or tests for the presence of free 

 complement, so that if an antigen is known, the antibody may be deter- 

 mined; or vice versa, by using a known antibody, the antigen may be 

 determined, the criterion in each instance being whether complement is 

 or is not bound or " fixed," a fact that is determined by the subsequent 

 addition of a hemolysin and its homologous corpuscles. 



The practical applications and technic of these reactions are given in 

 subsequent chapters. 



