558 MICRO-ORGANISMS AND DISEASE [chap. 



microbe, be this either a saprophyte or a pathogenic 

 microbe. 



Spontaneous or natural immunity. — The first who at- 

 tempted an explanation based on experiment was Metchni- 

 koff, who ascribed to the leucocytes, lymph- or white 

 blood-corpuscles the power of taking up, destroying, and 

 neutralising the microbe introduced into the tissue and 

 thereby protecting the tissue and the body from infection, 

 inasmuch as the microbes thus destroyed cease to exist, 

 to multiply, and to produce their toxic effects. This view 

 was based on the fundamental observation of Metchnikoff 

 that when in a normal frog anthrax bacilli are introduced 

 into the dorsal lymph sac, leucocytes soon rush, as it were, 

 and are attracted to the place, eat up the bacilli, and thus 

 protect the animal against infection, preventing the bacilli 

 from living, growing, multiplying, and causing disease. 

 This process of " phagocytosis," as it was called, is therefore 

 an essential feature in natural immunity ; it is in the first 

 instance a purely mechanical process, effected by the amoeboid 

 movements and capabihty of the leucocytes to embody and 

 swallow up and digest and destroy the invading enemy. 

 In a large number of instances of known immunity — of 

 greater or lesser degree — Metchnikoff and his pupils have 

 sought and found this process of mechanical phagocytosis, 

 and have explained to their own satisfaction every case 

 of immunity of one or another animal or its tissues 

 against one or another kind of microbes, be they true 

 saprophytes or true parasites. 



This theory relies on the following facts : (i) leucocytes 

 are well known to be capable in the course of their amceboid 

 movements of embodying and swallowing particulate matter 

 (2) leucocytes generally accumulate at, i.e. are attracted to. a 

 locality into which foreign particles en masse are introduced 

 or injected, and (3) it is notorious that in cases of immunity 



