CHAPTER XII 



IMMUNITY 



THE CAUSES AND MECHANISM OF IMMUNITY 

 NATURAL immunity, as we have learned, is that re- 

 sistance to certain diseases possessed by certain species 

 of animals. 



It is dependent upon many different conditions and 

 circumstances. Certain profound differences in the 

 life processes of the different species of animals are suffi- 

 cient to account for many instances of natural immu- 

 nity. Thus, the immunity of cold-blooded animals to 

 most diseases of warm-blooded species may be ex- 

 plained upon this ground. The tissues of the immune 

 individuals do not present favorable soil and environ- 

 ment for the growth and development of the germs which 

 cause that particular disease. Just what the condi- 

 tions are which are detrimental to the activities of the 

 germ we are not able to determine to any great extent. 

 True, we are able in certain cold-blooded animals to 

 convert this immunity into susceptibility by placing the 

 animals in a warm place. Doubtless, by subjecting the 

 immune individual to the same environment, the same 

 food, and the same mode of living we might be able to 



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