546 IMMUNITY 



now and then the cat or dog is found suffering with the disease. 

 While it must be admitted that all species may be infected 

 with tuberculosis there is much evidence to show that certain 

 species are largely immune to it. 



It sometimes happens that individuals belonging to a sus- 

 ceptible species resist infection. These are spoken of as pos- 

 sessing natural immunity ; but this is not a satisfactory 

 explanation, for in such cases it is practically impossible to 

 state whether it is natural or acquired. In the present 

 state of our knowledge of this subject it seems better to 

 allow such cases to remain as examples of individual re- 

 sistance. 



It has also been found that in cases of marked natural 

 immunity the resistance can be overcome and the animal 

 infected by changing its normal physical condition. Thus 

 fowls that are naturally immune to anthrax are said to be 

 made susceptible to its bacteria by reducing their temperature 

 by immersing them in cold water. Charrin and Roger found 

 that fatigue would lessen the resistance of white rats to the 

 same disease. Gibier found that frogs kept at a temperature 

 of 37" C. were susceptible to anthrax. 



It has been stated that sewer and other poisonous gases 

 predisposed animals to infection. Abbott concluded, after a 

 careful experiment of exposing rabbits to sewer gas and gases 

 of putrid meat, that they did not lessen the resistance of the 

 animals to infection. Diet and drugs have been reported to 

 have been successfully used in reducing the resistance to 

 infection. The removal of the spleen has been thought by 

 some experimenters to increase the susceptibility but others 

 have reported different results. 



Natural immunity usually persists under ordinary condi- 

 tions throughout life and in that respect it is much more per- 

 manent than artificial immunity. 



§ 442. Explanation of natural immunity. There are 

 a number of explanations of this phenomenon. In brief they 

 deal with the supposed actions (i) of the cells of the body, and 

 (2) of the serums or humors. 



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