CHAPTER X 

 IMMUNOLOGY. IMMUNITY AND IMMUNIZING AGENTS 



i. Introduction. Immunity from disease and susceptibility to disease 

 are relative terms. By immunity is meant the power or ability of the 

 living body to resist or prevent the successful invasion by the agent or 

 agents of infection, whether these agents are of vegetable or of animal 

 origin, whether they are the organisms themselves, as bacteria, amebse, 

 yeast cells, nematodes, etc., or the substances produced by them, as 

 toxins, ptomaines, albumins, toxalbumins, putrescins, venoms and 

 enzymes. The body resistance to the action of chemicals and chemical 

 poisons is generally not classed as immunity, although it is a closely 

 related phenomenon and cannot well be omitted from the discussion. 



We know that there is great variation in the immunity of the indi- 

 viduals of the same species to the various agents of infection as well 

 as to the action of the purely chemical poisons of non-living or inorganic 

 origin. A number of individuals exposed to the same infection do not 

 all take the disease. A number of individuals receiving the same dose 

 of poison are not all affected in the same degree or in the same way. It 

 has been known for a long time that the successful invasion of certain 

 infections as small-pox, typhoid, measles, mumps, whooping-cough, etc., 

 immunizes the individual against subsequent attacks. It has also been 

 known for ages that the animal organism may resist gradually increasing 

 doses of highly toxic substances, as arsenic, opium, morphine, tobacco, 

 and alcohol. Even more remarkable is race immunity. Man is immune 

 to most of the diseases of the lower animals, as hog cholera, chicken 

 cholera, and on the other hand most animals cannot be successfully 

 infected by the human diseases, such as measles, mumps, whooping- 

 cough, scarlet fever, and yellow fever. Closely related species may 

 display remarkable immunity differences. For example, field mice 

 are very susceptible to glanders, whereas the common house mouse is 

 quite immune. Jersey cows are less liable to bovine tuberculosis than 

 Holsteins. The Yorkshire breed of swine is less susceptible to hog 

 erysipelas than are other breeds. 



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