130 THE DIFFERENT TYPES OF IMMUNITY 



animals, like the frog, which is naturally immune to infection with 

 the anthrax bacillus, but which loses its resistance when kept at a 

 temperature at which the organism will normally develop outside 

 of the body. Conversely it has been noted that frogs which under 

 natural conditions, i. e., at low temperature, readily fall a prey to 

 infection with the bacillus ranicida, are immune to the same organ- 

 ism if kept at a temperature of 25 C. Of the same order, no doubt, 

 is the immunity of chickens to anthrax, which disappears when the 

 animals are kept immersed in water of 25 C., their normally h'gh 

 temperature being reduced in this manner. In cases such as these 

 the modus operand* of the temperature changes upon immunity 

 or infection seems relatively simple, while in others it is certainly 

 of a more complex order. 



It is thus a well-known fact that man and other animals after expo- 

 sure to cold are more prone to infection with a number of different 

 organisms, which find their optimum condition for growth at the 

 normal temperature of the body. The underlying causes of the 

 change in resistance in such cases are apparently different, but what 

 the mechanism is we do not know. We can readily imagine, however, 

 that functional disturbances may be set up in the macroorganism 

 by the cold which in some manner operate to the advantage of the 

 microorganism. It is not excluded, of course, that in the instances 

 of immunity mentioned above, something similar may not also be 

 operative, but the simple explanation that has been offered cannot 

 be overlooked. 



Athreptic Immunity. In other cases the resistance to infection 

 may be referable to the existence of unfavorable conditions of 

 nutrition. A number of observations have taught us that certain 

 organisms require certain specific foodstuffs for their development, 

 in addition to others which are necessary to all forms of life of that 

 order, and unless these are present, successful growth cannot take 

 place and immunity would accordingly result. Immunity of this 

 type is spoken of as athreptic immunity. 



Ehrlich first suggested this term to denote the peculiar behavior 

 of mouse cancer when transplanted into rats. At first active growth 

 takes place, so that at the end of eight to ten days the size of the 

 tumor does not differ from control tumors in mice. After that, 

 however, further growth ceases and resorption takes place. If, 

 now, i. e., at a time when active growth no longer occurs in rat A a 



