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 

 defensive reactions become, until it may no longer be possible 

 accurately to analyze them. 



Some have endeavored to refer all of the phenomena of im- 

 munity 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 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 preventing the production of the toxin to which the 

 frog was susceptible. 



Immunity must not be conceived as something inseparably 

 associated 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 bacterio- 

 toxins 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. 



