CHAPTER XXVI. 



THEORIES OF IMMUNITY. 



Pasteur and the bacteriologists of his time discovered 

 that bacteria cease to grow in artificial culture media after 

 a time, because of the exhaustion of the food material in 

 some cases and because of the injurious action of their own 

 products in other instances. These facts were brought for- 

 ward to explain immunity shortly after bacteria were shown 

 to be the cause of certain diseases. Theories based on these 

 observations were called (1) "Exhaustion Theory'' of Pasteur, 

 and (2) " Noxious Retention Theory'' of Chauveaii respec- 

 tively. The fact, soon discovered, that virulent pathogenic 

 bacteria are not uncommonly present in perfectly healthy 

 animals, and the later discovery that immunity may be 

 conferred by the injection of dead bacteria have led to the 

 abandonment of both these older ideas. The (3) " Unfavor- 

 able Environment" theory of Baumgartner, i. e., bacteria do 

 not grow in the body and produce disease because their 

 surroundings are not suitable, in a sense covers the whole 

 ground, though it is not true as to the first part, as was 

 pointed out above, and is of no value as a working basis, 

 since it offers no explanation as to what the factors are 

 that constitute the ''unfavorable environment.'' Metchnikoff 

 brought forward a rational explanation of immunity with 

 his (4) ''Cellular or Phagocytosis Theory." As first pro- 

 pounded it based immunity on the observed fact that cer- 

 tain white blood corpuscles, phagocytes, engulf and destroy 

 bacteria. Metchnikoff later elaborated the original theory 

 to explain facts of subsequent discovery. Ehrlich soon after 

 published his (5) "Chemical or Side-chain Theory," which 

 seeks to explain immunity on the basis of chemical substances 

 in the body which may in part destroy pathogenic organisms 

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