CHAPTER II.* 

 IMMUNITY AND SUSCEPTIBILITY. 



GENERAL. 



DEFINITION. At the outset it is necessary to have a clear understand- 

 ing of what is meant by the terms immunity and susceptibility. By 

 immunity is understood the resistance which an animal 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, character- 

 izes what is known as susceptibility. Throughout the animal kingdom and 

 also among the plants there is a great variation in the different species 

 in the immunity and susceptibility to various diseases. Immunity bears 

 no relation to the contagiousness of a disease and is only applied as a 

 rule to strictly infectious diseases and not metabolic diseases. 



HYPERSUSCEPTIBILITY OR ANAPHYLAXIS. It has been shown within 

 the last few years that animals and man are occasionally hypersusceptible 

 to certain proteins. For example, there are individuals who are seriously 

 poisoned by the ingestion of eggs, pineapples and strawberries. It has 

 been demonstrated that an animal may be sensitized to almost any protein 

 by first injecting a dose and then, after a period of eight to thirteen days, 

 may be seriously intoxicated, if not killed, by the injection of a very slight 

 dose of the same protein. The proteins of the bacterial cells have been 

 shown to act in the same way. Animals injected as above mentioned are 

 rendered hypersusceptible to the protein in question. Furthermore, as 

 referred to above, individuals may be naturally hypersusceptible. The 

 manner of the original sensitization in these cases is not known. As 



* Prepared by E. F. McCampbell. 



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