Complement Fixation 139 



We are unable experimentally to accomplish these prerequisites, 

 therefore are not in the position to accurately apply bacteriolytic 

 serums in practice. 



COMPLEMENT FIXATION 



In 1901 Bordet, while investigating the nature of the comple- 

 mentary substance, made a discovery that has now become of great 

 importance, that is, the " Bordet-Gengou phenomenon," or, as it 

 is now known, the " fixation of the complement." His method of 

 procedure was as follows: Blood-corpuscles were sensitized with 

 appropriate amboceptors and then treated with freshly-drawn nor- 

 mal serum. Hemolysis resulted. If now he added to the mixture 

 some sensitized blood-corpuscles of a different species, they did not 

 hemolyze. Clearly, the complement had been used up in the first 

 hemolysis. 



He next found that if, instead of employing blood-corpuscles for 

 the first test, he used sensitized bacteria i.e., bacteria treated with 

 an immune serum containing the amboceptors appropriate for effect- 

 ing their solution the complement would similarly be used up, 

 " fixed," so that when he subsequently added sensitized red blood- 

 corpuscles there was no hemolysis. 



This reaction was naturally quantitative, the result as described 

 depending upon the fact that no more complement (normal serum) 

 was used in the original hemolysis or bacteriolysis than was necessary 

 and so none left "unfixed" to effect the lysis or solution of the second 

 factor introduced. 



Bordet interpreted his results as indicating that there was only 

 one complementary or solvent substance, and though Ehrlich sub- 

 sequently published what he looked upon as proofs to the contrary, 

 the opinion of Bordet prevails. 



In addition, however, Bordet's experiments have been of practical 

 use. As affording a means of quantitative experimentation they 

 have enabled investigators to measure the quantity of complement 

 in normal bloods and in immunized bloods, and so led to the discovery 

 that for each kind of animal and for each individual animal the 

 complement is subject to very little variation. In the course of 

 some three years they were followed by the investigations of Neisser 

 and Sachs upon antigens, and made to subserve the useful purpose 

 of recognizing and differentiating antigenic substances. Thus, 

 when a certain antibody and its complement are combined they can 

 only attach themselves to the particular specific antigen by which 

 the antibody has been developed. But, what is still more important, 

 they have led to the invention of methods by which the presence of 

 specific amboceptors may be determined where they are suspected, 

 and so have made possible means of arriving at a correct diagnosis 

 in certain obscure cases of disease in man. 



The most important of these measures is the Wassermann reac- 



