SERUM DIAGNOSIS 153 



presence of specific antibodies. In order to ar- 

 rive at a clear perception of the phenomena of 

 complement fixation, it is essential that we first 

 have a clear idea of the nature of the substances 

 involved in the reaction. (1) ANTIGEN is a sub- 

 stance which has the power of inducing in the 

 animal body, after one or more inoculations, the 

 formation of antibodies of the amboceptor type. 

 Antigens comprise enzymes, toxins, and protein 

 bodies including bacterial proteins of bacterial 

 vaccines. (2) An ANTIBODY is one of the con- 

 stituents of the blood of animals rendered im- 

 mune by inoculation and exerting a specific an- 

 tagonistic influence on the substance under 

 whose influence it was formed that is, the an- 

 tigen. Antibodies include antitoxins, agglutin- 

 ins, precipitins, and amboceptors (bacteriolys- 

 ins, hemolysins, and probably opsonins). (3) 

 COMPLEMENT is a ferment-like body normally 

 present in all sera when freshly drawn from the 

 body, but is very unstable and disappears grad- 

 ually on standing. Fresh serum may be ren- 

 dered free of complement or "inactivated" by 

 heating to 56 C. for one-half hour. Comple- 

 ment is the active element of lysis (dissolution 

 of cells or of antigen), but it can act only in con- 



