188 INFECTION AND RESISTANCE 



is no longer available for Complex II. If either of the reacting 

 parts, antigen or sensitizer, of Complex I are lacking the alexin is 

 left unfixed and free to react with Complex II. 



With this technique Bordet and Gengou were able to demonstrate, 

 by indirect experiment, the presence of specific sensitizers in the 

 sera of animals immunized with various bacteria, a fact which was, 

 of course, surmised but had been amenable to proof heretofore only 

 in the case of bacteria like the spirillum of cholera in which lysis 

 under the influence of immune serum and alexin could be directly 

 observed under the microscope. The practical possibilities of their 

 method were, of course, immediately apparent. By the use of a 

 known antigen specific sensitizers can be demonstrated in this way, 

 and, vice versa, in the presence of a known antibody, the method 

 will serve to identify the nature of a doubtful antigen. Thus bac- 

 terial differentiation can be carried out by adding to the suspected 

 bacteria, in emulsion, a small quantity of a known antiserum and 

 alexin, and determining whether or not the alexin has become fixed. 

 And, conversely, Bordet and Gengou 76 have more recently utilized 

 the method in support of their claim of the specific etiological im- 

 portance of the bacillus isolated by them from whooping cough, by 

 showing that the serum of children suffering from this disease 

 formed a specific alexin-fixing complex when treated with the ba- 

 cillus. 



The phenomenon of Bordet and Gengou thus found' rapid prac- 

 tical application in the diagnosis of a number of infectious diseases, 

 and has, of course, attained great clinical importance in the diag- 

 nosis of syphilis in the form of the "Wassermann" reaction and its 

 many modifications. Before discussing these practical features in 

 greater detail, however, it will be useful to discuss more particularly 

 the many important theoretical considerations which have followed 

 in the train of the complement-fixation phenomena. 



A year after the publication of Bordet and Gengou's paper 

 Gengou 77 made another fundamentally important observation by 

 showing that complement or alexin fixation was not limited to the 

 complexes of cellular antigens and their antibodies, but that the sera 

 of animals immunized with dissolved proteins (animal sera, etc.), 

 when brought together with their specific antigens, likewise formed 

 combinations which fixed alexin. Thus egg-white or dog serum, 

 brought together with "anti-egg- white" or "anti-dog" rabbit serum, 

 respectively, strongly fixed alexin, whereas neither the antigenic sub- 

 stances nor the antisera exerted such fixation alone. The interpre- 

 tation put upon this by Gengou was the following : "In sera obtained 

 by injecting rabbits with large doses of cow's milk, etc., there are, 

 in addition to the precipitins of Bordet and Tschistovitch, substances 



76 Bordet and Gengou. Ann. de VInst. Past., 1906. 



77 Gengou. Ann de VInst. Past., 16, 1902. 



