Acquired immunity against micro-organisms 275 



the hypothesis just mentioned is of importance from a 

 i;eneral point of view it must be supported by adequate proof. It 

 ras during the course of the vaccination of rabbits against tlie 

 dcro-organism of the pneumo-enteritis epidemic at Gentilly that 

 was first able^ to assure myself of its accuracy. I noticed that 

 some of these rabbits, although vaccinated, ultimately succumbed 

 to pyaemia, set up solely by this micro-organism. They were con- 

 sequently not refractory against the disease, and yet their blood 

 serum, when injected into normal rabbits along with an absolutely 

 fatal dose of micro-organisms, was found to be highly protective. 

 This observation drove me to the conclusion that the protective 

 X30wer is not a function of immunity and cannot be received as a 

 measure of this immunity. Analogous facts have since been demon- 

 strated in certain other cases. Thus, Pfeifier^ on several occasions has [289] 

 found that guinea-pigs, highly immunised against the cholera vibrio, 

 have succumbed after the injection of a moderate quantity of these 

 organisms. "On post-mortem examination of these cases living 

 vibrios were found, in the peritoneal cavity, sometimes in considerable 

 numbers ; and yet minimal doses of the heart blood given to normal 

 guinea-pigs caused in these animals a very marked breaking down of 

 the vibrios." Alongside these facts may be placed others, described 

 in the preceding chapter, of well-immunised animals dying from infec- 

 tion, after they had been weakened by opium, cold, or other lowering 

 agent. It is clearly seen, then, that for the manifestation of acquired 

 immunity it is necessary that the reaction of the living cell elements 

 should take place without let or hindrance. When this reaction fails, 

 the possession of even great protective power is insufficient to prevent 

 the immunised animal from contracting a fatal infection. 



If, in acquired immunity against micro-organisms, it is really the 

 cell defence which plays the most important part, we can readily 

 imagine cases where it by itself can confer immunity without calling 

 in the co-operation of the protective power of the fluids. When in 

 this connection we study the resistance of an animal against various 

 pathogenic organisms, we note, first of all, the very great variability 

 that exists in the production of the acquired humoral properties. In 

 certain cases, as in vaccination against vibrios or typhoid bacilli, the 

 serum very readily becomes not only protective, but agglutinative 

 and fixative. In other cases these properties develop with difficulty 



1 An7i. de I' Inst. Pasteur, Paris, 1892, t. vi, p. 300. 

 * Ztschr.f. Hyg., Leipzig, 1895, Bd. xix, S. 82. 



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