CHAPTER IV. 

 IMMUNITY. 



IMMUNITY is ability to resist infection. It is the ability 

 of an organism to successfully antagonize the invasive 

 powers of bacteria, or to annul the injurious properties of 

 their products. The mechanism of immunity is compli- 

 cated or otherwise according to the necessity of the case. 

 When the invasive action of non-toxicogenic bacteria is 

 to be overcome, 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 combining 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. The reactions of immunity are, therefore, 

 comparatively simple or exceedingly complex, according to 

 the conditions involved, and the more involved these con- 

 ditions become, the more complicated the defensive reac- 

 tions become, until it may no longer be possible 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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