CHAPTEE XXVI 



1900. Immunity — Natural Immunity — ^Artificial Immunity. 



For centuries tte question of immunity has occupied 

 the human mind because the prevention of disease 

 has ever been one of the greatest preoccupations of 

 Man. Savages had already observed that man can 

 become refractory to the venom of serpents, either 

 through a sHght bite or by the application of certain 

 preparations of that venom on scarified skin. It 

 was also a popular and very ancient notion that the 

 contact of a sUghtly scratched hand with the pustules 

 of cow-pox conferred immunity against human small- 

 pox. It was on this observation that Jenner founded 

 his method of antivariolic vaccination. The latter, 

 in its turn, suggested to Pasteur the idea of attempting 

 antimicrobian vaccinations. Having ascertained that 

 old cultures of chicken cholera, previously very viru- 

 lent, had become harmless, he wondered whether they 

 had become a vaccine and proved by experiment 

 that they had. That led him to the principle of the 

 attenuation of viruses and to that of vaccination 

 by attenuated microbes. Thus the problem of the 

 mechanism of immunity was stated. 



The first theories propounded on the subject con- 

 cerned the humors. Pasteur supposed that im- 

 munity was due to the absorption, by the vaccinating 

 microbes, of certain nutritive substances in the humors, 



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