Acquired immunity against micro-organisms 253 



distinguishes these two categories of immunity. Now, the fixative 

 is found in the serum of perhaps all cases of acquired immunity. 

 Bordet and Gengou have studied it by the method already mentioned 

 (Chap. VII.). A certain quantity of micro-organisms of various species 

 is introduced into the serum. If the cytases, present in the serum 

 when the experiment was commenced, ultimately disappear from it, 

 it indicates that this ferment has been absorbed by the bacteria, 

 thanks to the fixative, which consequently should be found in the 

 serum under observation. The presence or absence of the cytases 

 can be demonstrated by the production or absence of Pfeiffer's phe- 

 nomenon with vibrios. 



The application of this method enabled Bordet and Gengou 1 

 to satisfy themselves that the serum of animals immunised against 

 several species of bacteria (plague bacillus, typhoid bacillus, bacillus 

 of swine erysipelas, first anthrax vaccine, and Proteus vulgaris), really 

 contains an appreciable quantity of fixative. It may, then, be ac- 

 cepted that the production of this substance is fairly constant in 

 acquired immunity against bacteria, and that it constitutes one of the 

 most important factors in such immunity. 



The question has been raised : What is the nature of the substance 

 to which the name of fixative is given? Pfeifler and Proskauer 2 

 have attempted to solve this question by making use of a serum which 

 acts against the cholera vibrio and which they obtained by vaccinating 

 animals with this vibrio. They carried out a long series of experi- 

 ments which led them to the conclusion that this substance, which 

 they term " cholera antibody," cannot be identified with any of the 

 albuminoid substances of the serum. Further, the fixative is not 

 represented by any of the salts or extractive substances of the serum, 

 because these substances dialyse easily, whereas the cholera antibody 

 does not pass through the dialysing membrane. The fixative is wholly 

 precipitated by alcohol, and is regarded by Pfeifier and Proskauer as 

 belonging to the category of soluble ferments, an opinion which is 

 certainly shared by many other investigators. 



What is it that communicates to this ferment its remarkably [267] 

 specific character? Without being able to give a precise answer 

 to this question, the authors just cited point out the analogy that 

 exists between the cholera antibody and the soluble ferments of 

 yeasts which have been studied by Emil Fischer. Some of these 



1 Ann. de I'Inst. Pasteur, Paris, 1901, t. xv, p. 289. 



2 Centralbl.f. Bacterial, u. Parantenk., Jena, 1896, l te Alt, Bd. xix, S. 191. 



