72 INFECTION AND IMMUNITY 



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, how- 

 ever, 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 this theory. 



The recorded experimental evidence leads us to 

 the conclusion that natural immunity is partly due 

 to germicidal substances present in the blood-serum, 

 which have their origin in the leucocytes, and are 

 soluble only in an alkaline medium ; that local infec- 

 tion is usually resisted by an afflux of leucocytes to 

 the point of invasion, which to some extent serve to 

 protect the individual from disease germs, by their 

 direct action as " phagocytes"- that is, by picking 

 up and destroying the invading parasites. These 

 agencies, together with conditions relating to body 

 temperature, and the chemical constitution of the 

 fluids and tissues of the body constitute the principal 

 factors upon which natural immunity depends. 



