50 ESSAYS ON BACTERIOLOGY. 



cry of Metschnikoff, wMcli explains immunity thus: 

 that the cells, especially the leucocytes, have conferred 

 upon them, as the result of the first attack, the special . 

 property of subsequently devouring and destroying 

 the same germs. Fourth, the theory of the germi- 

 cidal power of the blood serum. It is known that 

 certain animals are naturally unsusceptible to certain 

 diseases, and that the introduction of their blood se- 

 rum into susceptible animals may confer upon them a 

 more or less lasting power of resisting those germs to 

 which the first animal was naturally immune. This 

 is the theory upon which was based the goat blood and 

 dog blood treatment of tuberculosis. 



Of these theories Frsenkel observes that there is 

 probably a truth in all, and that artificial immunity is 

 not acquired by one regular process, but sometimes in 

 this manner, sometimes in that, and that it is quite 

 possible and even probable that causes are at-work 

 which are as yet unknown, and which future investi- 

 gators are destined to discover. But whatever the 

 manner of action, the primary fact remains that in 

 certain diseases the growth of the germs in the body 

 leaves it protected against a subsequent invasion of 

 the same germ; and further, that this acquired im- 

 munity is brought about throiigh the agency of the 

 new chemical bodies evolved during the germ growth. 



Naturally, the knowledge of these facts has given 

 rise to an extensive series of experiments, all having 

 for their object the imitation of nature in securing 



