262 INFECTION AND IMMUNITY 



The Determination of Antibodies in Sera by Complement Fixation. 



The principle of complement fixation, discovered by Bordet and Gengou * 

 in 1901, has been utilized both in bacteriological investigations, and in 

 practical diagnosis for the determination in serum of the presence of 

 specific antibodies. The reaction depends upon the fact that when an 

 antigen, i.e., a substance capable of stimulating the formation of anti- 

 bodies, is mixed with its inactivated antiserum, in the presence of com- 

 plement, the complement is fixed by the combined immune body and 

 antigen can no longer be found free in the mixture. If such a mixture 

 is allowed to stand at temperature for an hour or more, and to it is 

 then added an emulsion of red blood cells together with inactivated 

 hemolytic serum, no hemolysis will take place, since there is no free 

 complement to complete the hemolytic system. If, on the other hand, 

 the original mixture contains no antibody for the antigen used, the 

 complement present is not*fixed and is available for the activation of 

 the hemolytic serum later added. 



The reaction thus depends upon the fact that neither antigen 2 alone, 

 nor amboceptor (antibody) alone, can fix complement, but that this 

 fixation is carried out only by the combination of antigen plus ambo- 

 ceptor. Any specific can be determined by this method, provided the 

 homologous antigen is used; and vice versa, by the use of a known anti- 

 body a suspected antigen may be determined. 



When testing immune sera for antibodies given rise to in man or 

 animals by microorganisms which can be cultivated, either the whole 

 bacteria or extracts of the bacteria may be used as an antigen. 



For the diagnosis of syphilis by this method, in the so-called " Wasser- 

 mann reaction," the antigen employed was originally obtained by the 

 extraction of syphilitic organs, in which free syphilitic antigens, i.e., 

 uncombined products of Spirochaete pallida, were assumed to be present. 



It has been more recently shown, however, that the Wassermann 

 reaction is not specific in any sense of the word, and that suitable anti- 

 gens can be produced by the alcoholic extraction of lipoids from the 

 normal organs of many animals and man. 



Bacterial extracts for complement-fixation can be made in various 

 ways. The use of thick salt solution suspensions of the cultures them- 

 selves is not advisable because of the anticomplementary action of 

 such suspensions. Good bacterial antigens can be produced by cen- 



1 Bordet and Gengou, Ann. de 1'inst. Pasteur, xv, 1901. 



2 Bordet and Gay, Ann. de 1'inst. Pasteur, xx, 1906. 



