IMMUNITY. 2 1 



of amoebae, any particles, organic or inorganic, which come in 

 their way. And it requires no great stretch of credulity to 

 believe that they may, like an amceba, digest and assimilate the 

 protoplasm of the captured bacterium, thus putting an end to the 

 possibility of its doing any harm. 



" In the case of a pathogenic organism we may imagine 

 that, when captured in this way, it may share a like fate if the 

 captor is not paralyzed by some potent poison evolved by it, or 

 overwhelmed by its superior vigor and rapid multiplication. 

 In the latter event the active career of our conservative white 

 corpuscles would be quickly terminated and its protoplasm 

 would serve as food for the enemy. It is evident that in a 

 contest of this kind the balance of power would depend upon 

 circumstances relating to the inherited vital characteristics of 

 the invading parasite and of the invaded leucocyte." 



This explanation is now very commonly spoken of as the 

 " Metschnikoff theory," although, as shown by the above 

 quotations, it was clearly stated by the writer several years 

 (1881) before Metschnikoff' s first paper (1884) was published. 

 Metschnikoff has, however, been the principal defender of this 

 explanation of acquired immunity, and has made extensive 

 and painstaking researches, as a result of which many facts 

 have been brought to light which appear to give support to 

 the present writer's hypothesis, the so-called Metschnikoff 

 theory. 



The time at my disposal will not permit me to review the 

 experimental evidence for and against the view that phagocytosis 

 is the principal factor in protecting animals from invasion by 

 pathogenic bacteria. The conclusion which I have reached is 

 stated in my recently published work on Immunity, Protective 

 Inoculation, and Serum- Therapy as follows : 



"The experimental evidence submitted, considered in con- 

 nection with the extensive literature relating to 'phagocytosis,' 

 leads us to the conclusion that natural immunity is due to a 

 germicidal substance present in the blood-serum which has its 

 origin (chiefly, at least) in the leucocytes, and is soluble only 

 in an alkaline medium ; and that local infection is usually 

 resisted by an afHux of leucocytes to the point of invasion, 



