172 LIFE OF ELIE METCHNIKOFF 



Therefore, in the natural immunity of unicellular 

 beings, two fundamental elements may already be 

 observed : sensitiveness and intracellular digestion. 

 No researches have yet been made on the possibility of 

 conferring on protozoa an artificial immunity against 

 certain pathogenic microbes and their poisons. But 

 unicellular beings, insensible to microbian poisons, are 

 the reverse to many chemical substances which, in their 

 normal life, they have no opportunity of ingesting. 



It has been proved by experiment that, against 

 many of those chemical substances, an artificial im- 

 munity may be given to the protozoa by accustoming 

 them gradually. Very diluted solutions are added at 

 first to the medium in which they live and, by gradu- 

 ally concentrating those solutions, an artificial im- 

 munity is conferred ; the negative chimiotaxis becomes 

 positive, allowing the protozoa to absorb and digest 

 the poison, now become a food. 



Habit is therefore the fundamental condition of 

 artificial immunity ; it must be that also of immunity 

 naturally acquired. Having accidentally digested 

 enfeebled microbes or having suffered an attack of 

 disease, the unicellular being becomes accustomed to 

 a stronger virus and becomes immune against it. 

 The fact that so many unicellular beings have become 

 thus accustomed is therefore connected with their 

 sensitiveness and their digestion. Accordingly, 

 sensitiveness, habit, and digestion are the funda- 

 mental factors of the mechanism of immunity in 

 protozoa ; this immunity thus indisputably belongs 

 to the category of purely cellular phenomena. 



Having arrived at this conclusion, Metchnikoff 

 thought that the same mechanism of immunity must 

 be found in other primitive and analogous cells, such 



