CHAPTER IV. 

 IMMUNITY. 



IMMUNITY is ability to resist infection. It is the ability 

 of an organism successfully to antagonize the invasive 

 powers of parasites, or to annul the injurious properties of 

 their products. The mechanism of immunity is compli- 

 cated or otherwise according to circumstances. When the 

 invasive action of non-toxicogenic bacteria is to be over- 

 come, certain reactions, mostly on the part of the phagocytic 

 cells, are called into action; when the toxic products of 

 bacteria are to be deprived of injurious effects, the reaction 

 seems to take place between the toxin and certain com- 

 bining and neutralizing substances contained in the body 

 juices; when bacterial invasion and intoxication are both 

 to be" antagonized, both mechanisms are engaged in the 

 defenses, comparatively simple or exceedingly complex, 

 according to the conditions involved. The more involved the 

 conditions of infection become, the more complicated the 

 defensive reactions become, until it may no longer be pos- 

 sible accurately to analyze them. 



Some have endeavored to refer all of the phenomena of 

 immunity to the ability of the animal to endure the bacterio- 

 toxins, and have sought to relegate the reactions against 

 invasion to a subsidiary place. This is undoubtedly an 

 error, as the mechanisms are different and the prompt action 

 of one may make the action of the other unnecessary. 

 Metschnikoff* found that frogs injected with 0.5 c.c. of 

 cholera toxin died promptly, but that frogs injected with 

 cultures of the cholera spirillum recovered without illness. 

 This would suggest that the recovery of the infected frog 

 depended upon some defensive mechanism combating the 

 invasiveness of the bacteria and so preventing the produc- 

 tion of the toxin to which the frog was susceptible. 



Immunity must not be conceived as something insepar- 



* " Immunite dans les Maladies Infectieuses," Paris, 1901, p. 150. 



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