CHAPTER XIII. 



IMMUNITY AND HYPERSUSCEPTIBILITY. THEORIES 

 OF IMMUNITY. 



Immunity. Immunity is that condition of a living organism 

 which enables it to escape without contracting a disease when 

 fully exposed to conditions which normally give rise to that disease. 

 Immunity may depend upon many different factors, or upon 

 only one of a great variety. In general, we shall find that it 

 depends very largely upon those factors which we have already 

 considered in the preceding chapters, such as the possession of 

 anatomical structures or habits of life which render invasion by 

 the particular parasite impossible, or the possession of a body 

 structure, physically or chemically not adapted for the growth 

 of the particular disease virus, or the ability to harbor the particular 

 parasite as a commensal without suffering injury, or the ability to 

 react against the invading parasite and destroy it by phagocytosis 

 or by cytolysis, neutralize its poisons by antitoxins, or limit its 

 activity by encapsulation. Immunity is ordinarily considered 

 under two heads, Natural Immunity, or that present as a part of 

 the individual's birthright, and Acquired Immunity, that which 

 follows as the result of some experience of the individual. 



Immunity of Species. Natural immunity to certain diseases 

 is possessed by certain species of animals. Where the morphology 

 and physiology is quite different from that of the usual victims 

 of the disease, immunity might be expected. Thus cold-blooded 

 vertebrates, fish, amphibians and reptiles, are immune to many 

 diseases of mammals, apparently because of the different tem- 

 perature of their tissues. In other instances the difference 

 in resistance between two species of animals seems to be correlated 



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