494 THE TECHNIC OF COMPLEMENT-FIXATION REACTIONS 



PROTEIN DIFFERENTIATION BY COMPLEMENT FIXATION 

 The Determination of an Antigen by Complement Fixation. In the 



tests hitherto considered the antigens were known, and the suspected 

 antibodies sought for in the blood-serum or other body-fluid. In making 

 the reactions it was necessary to bring the serum to be tested into con- 

 tact with the antigen specific for the suspected antibody, in the presence 

 of complement, and at a suitable temperature. At the end of an hour 

 the mixture was tested for free complement by adding hemolytic ambo- 

 ceptor and red blood-corpuscles. This order may be reversed, and with 

 a known antibody the suspected antigen may be detected. The anti- 

 gen to be detected, as in a solution of blood or bacterial extract, is 

 brought into contact with its specific antibody in the presence of com- 

 plement. At the end of an hour, at a suitable temperature, the mixture 

 is tested as previously for free complement, by adding corpuscles and 

 hemolytic amboceptors. Under proper conditions complement fixation 

 would indicate a specific reaction between the antibody and its antigen, 

 and thus serve to identify the latter. 



This method has clinical applications similar to those in which the 

 precipitin reaction is used : 



1. In the differentiation of blood-stains a solution of the stain con- 

 stitutes the unknown antigen. By furnishing a known antiserum the 

 antigen is detected, i. e., the animal from which the blood was derived 

 is ascertained. 



2. In the recognition and differentiation of meats. 



3. In the detection of bacterial antigens in the blood-serum of 

 patients, or with highly immune serums an unknown bacterial antigen 

 may be identified and the test employed as a means of differentiation 

 among bacterial species. 



4. Similar applications of the test may be made in the differentiation 

 of milks, seminal stains, and other albuminous substances. 



As compared with precipitin reactions, the complement-fixation test 

 is probably more delicate and reliable and easier of interpretation. The 

 technic of the latter method is, however, more complicated, and the 

 liability to error is greater unless the principles of complement fixation 

 in general are thoroughly understood and the importance of quantitative 

 factors is appreciated. 



1. Complement Fixation for the Identification of Blood-stains. The 

 application of the technic of complement fixation to the determination 

 of specific protein antigen, such as human or animal blood, was demon- 



