THE BACTERIOPHAGE AND IMMUNITY 281 



The immunity acquired as the result of a single injection of a 

 small quantity of bacteriophage culture is accompanied by the 

 appearance in the blood of a protective principle. The animal 

 which receives this blood enjoys a solid immunity, specific in 

 nature, and identical with that possessed by the animal which 

 received the immunizing injection of bacteriophage. The pro- 

 tective principle is probably an antitoxin. It is possible that 

 this new method of obtaining immunizing sera offers a means of 

 intervening in the course of a disease, even in cases where the 

 administration of the active culture of bacteriophage may be 

 without effect because of the previous acquisition of a resistance 

 by the bacterium. 



Our knowledge of the bacteriophage, a cultivable agent of 

 immunity, allows us to entertain the possibility of collective in- 

 tervention in epidemics. 



Whatever may be the epidemic (provided, of course, the agent 

 is known and cultivable) we have first the possibility of individual 

 vaccination by means of a single injection of a small quantity of 

 bacteriophage culture active for the causative bacterium. But 

 we have seen that the presence in the intestine of active ultrami- 

 crobes assures the protection of a susceptible individual. We 

 are then able to consider the possibility of collective immuniza- 

 tion of the population, for it would be easy to mix cultures of 

 the bacteriophage with the drinking water, especially in urban 

 centers. One might then be assured of an active bacteriophage 

 in the intestine of all susceptible individuals throughout the 

 critical period. The method offers no risk; the cultures can be 

 ingested without inconvenience in any quantity. 



In spite of the fact that I have specified at several times in 

 the course of this work that the experiments undertaken deal 

 only with antibacterial immunity in the susceptible individual, 

 I want in closing, in order to avoid confusion, to say a few words 

 on the subject of phagocytosis, for it would be strange if, speaking 

 of antibacterial immunity, I made no allusion to this mode of 

 defense. 



I do not oppose any of the conclusions of Metchnikoff touching 

 the role of phagocytosis in the natural immunity which character- 

 izes the refractory state. In acquired immunity also, Metchni- 



