PRACTICAL APPLICATIONS 423 



any complement-fixation method, but efforts to circumvent or ignore 

 them are likely to lead to errors in technic. A proper understanding and 

 appreciation of these factors constitutes the basis for reliable work, 

 whereas less essential details may be altered to conform to the ideas and 

 convenience of the individual worker. 



PRACTICAL APPLICATIONS 



It will be understood, therefore, that complement-fixation reactions 

 may serve two primary purposes : 



1. With a known antigen, the antibody may be found. This is the 

 usual order in diagnostic tests. For example, in the reaction for syphilis 

 the antigen is furnished and the antibody sought for in the body-fluids. 

 So specific has this test proved in the diagnosis of this disease that a 

 positive reaction secured with a proper technic is regarded as strong 

 evidence of the existence of lues, even though the primary lesion had 

 occurred years before and the person is at the time in apparent good 

 health. In the gonococcus fixation test and other tests of a similar nature 

 the antigen is known and is furnished, and the antibody is tested for in 

 the serum. 



2. With a known antibody the corresponding antigen may be found. 

 This order of events has less practical application, and is used principally 

 in the diagnosis of blood-stains and in the differentiation of proteins in 

 general. It is also used in making special bacteriologic investigations, 

 when an organism may be identified by specific complement fixation 

 with its known antibody serum. In these instances the antibody serum 

 is secured by immunizing rabbits with a known antigen, the immune 

 serum then being used for selecting the antigen in unknown substances 

 and mixtures. 



Complement-fixation methods have their greatest value, and are 

 probably best known, in the serum diagnosis of syphilis the biologic 

 syphilitic reaction of Wassermann, Neisser and Bruck, and Detre. 

 Although originally believed to be a direct application of the specific 

 Bordet-Gengou phenomenon of complement fixation, subsequent inves- 

 tigations have shown that the antigen need not be specific, in the sense 

 of containing the Spirocheta pallida, but that lipoidal substances in 

 general may serve as " antigen," the peculiar and specific character of 

 the reaction depending upon the nature of the antibody, which has a 

 strong affinity for lipoids, and in such a mixture is capable of absorbing 

 or fixing a considerable amount of complement. 



