122 LIFE OF ELIE METCHNIKOFF 



not attack virulent bacteria, they attacked and 

 rapidly digested attenuated bacteria. Moreover, he 

 observed a very active phagocytosis in refractory 

 animals and the reverse in sensitive ones. 



He thus came face to face with the question of 

 immunity. 



He approached it by a comparative examination 

 of the reaction of the organism of vaccinated rabbits 

 and of non-vaccinated ones, and ascertained that an 

 active phagocytosis was only manifested in a pre- 

 viously vaccinated organism. MetchnikofE explained 

 these facts by the theory that the phagocytes became 

 accustomed, gradually, through vaccination, to strive 

 against more and more virulent microbes. 



From that moment, immunity appeared to him as 

 being no other than this progressive hardening. He 

 published his researches in 1884 in Virchow's Archiv, 

 and impatiently awaited medical reviews, hoping to 

 find some answer, but the memoir passed unnoticed ; 

 the full significance of it had not been grasped. 



