CHAPTER XIII. 



IMMUNITY AND PROTECTIVE INOCULATION. 



§ 440. Immunity. In a broad sense, immunity is 

 "resistance to disease." The term, however, is usually 

 restricted to the infectious maladies and signifies a condition 

 of the individual which enables it to successfully defend itself 

 against the invasion of its tissues and organs' with the infecting 

 microorganisms or to resist the toxic effect of the invading 

 organisms should they gain entrance and multiply within the 

 body. While it usually applies to the action of pathogenic 

 bacteria, the protozoa are not excluded. It will be seen that 

 immunity is only relative; it is neither permanent nor con- 

 stant but varies with natural and artificial conditions. Accord- 

 ing to the process by which it is established in the individual, 

 immunity is recognized as natural or artificial. 



§ 441. Natural immunity. The term natural immunity 

 has been applied to that condition or ability possessed by some 

 races or species of animals which enables them to resist the 

 natural invasion of infecting organisms that attack other varie- 

 ties or species of animals. In so far as we know, it is a con- 

 dition inherent in the very nature of the individual, born with 

 it and transmitted to its offspring. 



There are a number of very striking examples of natural 

 immunity. The Algerian race of sheep are immune to natural 

 infection of anthrax, whereas other sheep are very susceptible 

 to it. The equine species is susceptible to glanders but the 

 ruminants are immune. Blackleg, which is very destructive 

 to cattle, does not attack horses, the carnivora or man. There 

 are, however, instances where there seems to be a general 

 immunity but where individuals are occasionally attacked. 

 Thus, it is not usual to find tuberculosis in the carnivora, but 



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