288 Chapter IX 



strations were given by Bordet 1 and Mesnil 2 with respect to strepto- 

 cocci and to the bacilli of swine erysipelas. We must, then, conclude 

 that we have here to do with a general law. Some experiments 

 made by de Nittis 3 might seem to indicate an exception to such a 

 law. He observed that anthrax bacilli when grown in the serum of 

 vaccinated pigeons lost a part of their virulence. It must not be 

 forgotten, however, that he grew his cultures under special con- 

 ditions ; the bacillus was grown for several days at 42 C., this in 

 itself being quite sufficient to bring about a certain attenuation of 

 virulence. 



The theory of the attenuating action of the body fluids, based on 

 the attenuation of the virus in the serum of vaccinated animals, can 

 no longer be maintained, as it is a well-established fact that the 

 serum, obtained outside the body, is a fluid differing in character and 

 properties from the plasma of the living animal. We have seen up 

 to what point this demonstration has shaken the theory of the 

 bactericidal action of the body fluids. 



It cannot be doubted that a micro-organism may undergo a certain 

 weakening in virulence, as well as in certain other functions, in the 

 body of the animal that has acquired immunity. But the question 

 must be put : Is this effect obtained as the result of humoral or of 

 cellular action ? As a general rule, exudations obtained from 

 vaccinated animals, and containing living micro-organisms, are found 

 to be virulent when inoculated directly into susceptible animals. 

 This fact was established by Pasteur 4 when he first carried out his 

 researches on acquired immunity against fowl cholera. He showed 

 that the exudations of vaccinated fowls set up a fatal disease in 

 [303] normal fowls, without there being the least evidence of any attenu- 

 ation of the micro-organism. The same applies to the Gentilly 

 cocco-bacillus and to the anthrax bacillus in a very great majority 

 of examples. De Nittis observed that the exudations of immunised 

 pigeons produced a fatal infection in the guinea-pig and in the mouse. 

 In the immunised guinea-pig, on the other hand, he found that the 

 exudations soon became innocuous for these animals. This alteration, 

 however, must be attributed not to the body fluids (which exhibit no 

 protective or attenuating power) but to the action of the cells. 



1 Ann. de I'Inst. Pasteur, Paris, 1897, t. xi, p. 177. 



2 Ann.de I'Inst. Pasteur, Paris, 1898, t. xn, p. 481. 



3 Ann. de I'Inst. Pasteur, Paris, 1901, t. xv, p. 769. 



4 Compt. rend. Acad. d. sc., Paris, 1880, t xc, p. 1033. 



