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 mech- 

 anism of immunity is complicated or otherwise according to cir- 

 cumstances. When the invasive action of non-toxicogenic bacteria 

 is to be overcome, certain reactions, mostly on the part of the phago- 

 cytic 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, comparatively simple or exceedingly com- 

 plex, according to the conditions involved. The more involved 

 the conditions of infection become, the more complicated the de- 

 fensive reactions become, until it may no longer be possible accu- 

 rately 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 difierent 

 and the prompt action of onejnay make the action of the other un- 

 necessary. Metchnikoff* found that frogs injected with 0.5 cc. 

 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 pre- 

 venting the production of the toxin to which the frog was susceptible. 



Immunity must not be conceived as something inseparably as- 

 sociated with infection. The reactions of the body toward bacteria 

 in the infectious diseases are identical with those toward other 

 minute irritative bodies, and the reactions toward bacteriotoxins 

 are identical with those toward other toxic substances, so that the 

 only way by which a satisfactory understanding of the phenomena 

 can be reached is by carefully comparing the reactions produced by 

 bacteria and their products with those produced by other active 

 bodies. 



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



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