CHAPTER XX. 



IMMUNITY. 



Introductory. — By immunity is meant non-susceptibility to a 

 given disease or to a given organism, either under natural con- 

 ditions or under conditions experimentally produced. The term 

 is also used in relation to the toxins of an organism. Immunity 

 may be possessed by an animal naturally, and is then usually 

 called natural immunity, or it may be acqiiired by an animal 

 either by passing through an attack of the disease or by artificial 

 means of inoculation. It has been shown that certain diseases 

 affect the lower animals but never occur in the human subject, 

 e.g. swine plague ; and, on the other hand, diseases such as 

 typhoid fever and cholera do not under natural conditions affect 

 any of the lower animals, so far as is known. That is to say, 

 man and the lower animals respectively enjoy immunity against 

 certain diseases, when exposed to infection under ordinary con- 

 ditions. From this fact, however, it does not follow that when 

 the organisms of the respective diseases are introduced into the 

 body by artificial methods of inoculation, pathological effects 

 will not follow. We have seen above, for example, that the 

 organisms of cholera and typhoid may artificially be made to in- 

 fect guinea-pigs, though they do not do so naturally. Immunity 

 may thus be of very varying degrees, and accordingly the use 

 of the term has a correspondingly relative significance. Such a 

 thing as absolute immunity is scarcely known, just as we have 

 seen in the case with absolute susceptibility. This is not only 

 true of infection by bacteria, but of toxins also ; — when the re- 

 sistance of an animal to these is of high degree, the resistance 

 may be overcome by a very large dose of the toxic agent. For 

 example, the common fowl may be able to resist as much as 

 20 c.c. of powerful tetanus toxin, but on this amount being ex- 

 ceeded may be affected by tetanic spasms (Klemperer). On 

 the other hand, in cases where the natural powers of resistance 



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