SERUM DIAGNOSIS 155 



fixed by the interaction of antigen and antibody 

 in such a way that it can no longer be found 

 free in the mixture. If such a mixture is al- 

 lowed to stand at a suitable temperature for a 

 certain period and then to it is added an emul- 

 sion of red cells, together with hemolytic am- 

 boceptor, no hemolysis will take place since 

 there is no free complement available to com- 

 plete the hemolytic system. If, on the other 

 hand, the first mixture contained no antibody 

 for the antigen used, the complement present is 

 not fixed and is free to act with the aid of the 

 hemolytic amboceptor upon the red cells, caus- 

 ing dissolution or hemolysis. Thus, the reac- 

 tion depends upon the fact that neither the an- 

 tigen alone nor antibody alone can fix comple- 

 ment, but that this fixation is carried out only 

 by the combination of antigen plus antibody. 



Practical Application The Bordet-Gengou 

 phenomenon of complement fixation is used in 

 the diagnosis of various infectious diseases, 

 notably gonorrhea and syphilis ; in the differen- 

 tiation of proteins ; in the standardization of an- 

 tibacterial sera ; and in establishing the etiology 

 of infectious diseases. The phenomenon has 

 been extensively used by Wassermann and 



