20 BIOLOGICAL LECTURES. 



Explanation of Natural Immunity. 



We have now to inquire upon what the natural immunity 

 depends which enables the healthy animal body to resist inva- 

 sion by the destructive agents referred to. 



Phagocytosis. — In my chapter on " Bacteria in Infectious 

 Diseases," in Bacteria, published in the spring of 1884, but 

 placed in the hands of the publishers in 1883, I say : — 



" It may be that the true explanation of the immunity 

 afforded by a mild attack of an infectious germ disease is to 

 be found in an acquired tolerance to the action of a chemical 

 poison produced by the micro-organism, and consequent ability 

 to bring the resources of nature to bear to restrict invasion by 

 the parasite." 



In the same chapter the resources of nature supposed to be 

 brought to bear in restricting invasion by the parasite are 

 referred to as follows : — 



" If we add a small quantity of culture fluid containing the 

 bacteria of putrefaction to the blood of an animal withdrawn 

 from the circulation into a proper receptacle and maintained in 

 a culture oven at blood-heat, we will find that these bacteria 

 multiply abundantly, and evidence of putrefactive decomposi- 

 tion will soon be perceived. But if we inject a like quantity 

 of the culture fluid, with its contained bacteria, into the cir- 

 culation of a living animal, not only does no increase and no 

 putrefactive change occur, but the bacteria introduced quickly 

 disappear, and at the end of an hour or two the most careful 

 microscopical examination will not reveal the presence of a 

 single bacterium. This difference we ascribe to the vital 

 properties of the fluid as contained in the vessels of a living 

 animal, and it seems probable that the little masses of proto- 

 plasm known as white blood-corpuscles are the essential histo- 

 logical elements of the blood, as far as any manifestation of 

 vitality is concerned. The writer has elsewhere {1881) szig- 

 gested that the disappcaratice of the bacteria from the circidation, 

 in the experiments referred to, may be effected by the white 

 corpjiscles, which, it is well known, pick up, after the manner 



