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 factbrs 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 abihty to harbor the par- 

 ticular 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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