CHAPTER II* 

 IMMUNITY AND SUSCEPTIBILITY 



GENERAL 



DEFINITION. A clear understanding as to what is meant by the 

 terms immunity and susceptibility is of fundamental importance. 

 By immunity we mean the resistance which ah animal or plant body 

 possesses to the etiological microorganisms of an infectious disease and 

 to the disease itself. The name has been adapted from the Latin 

 immunis which meant a person who was free or exempt from public 

 duties and later, one who was exempt from the action of poisons. 

 Briefly stated, immunity is resistance to disease. It results commonly as 

 a natural termination of the process of self-healing in many infectious 

 diseases. The absence of such resistance, which may be total or partial, 

 characterizes what is known as susceptibility. Throughout the animal 

 kingdom and also among the plants there is a great variation in the 

 immunity and susceptibility in the different species to the various dis- 

 eases. Immunity bears no relation to the contagiousness of a disease 

 and the term is only applied as a rule to strictly infectious diseases and 

 not to metabolic diseases. 



HYPERSUSCEPTIBILITY OR ANAPHYLAXIS. It has been shown that 

 animals and man are occasionally hypersusceptible to certain proteins. 

 For example, there are individuals who are always seriously poisoned 

 by the ingestion of eggs, pineapples, and strawberries. Certain indi- 

 viduals when injected with diphtheria or tetanus antitoxin which is 

 carried in horse serum are seriously intoxicated and occasionally die. 

 In such instances the proteins of the horse serum and not the antitoxin 

 are responsible. It has been demonstrated that an animal may be 

 sensitized or rendered hypersusceptible to almost any protein by first 

 injecting a minute amount of the protein and then, after a period of at. 

 least eight to thirteen days, may be seriously intoxicated, if not killed, 

 by the injection of a slightly increasing dose of the same protein. The 



* Prepared by E. F. McCampbell. 



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