IMMUNITY. 321 



antitoxin-immunity obtained with serum, ani- 

 mals seem to be immunized so quickly that 

 any participation of the body-cells in the pro- 

 cess appears to be out of the question. Pro- 

 tective substances are apparently introduced 

 directly with the serum ; they must therefore 

 act almost immediately, and like all foreign 

 substances are also soon eliminated. Let us 

 seek to discover therefore the nature of these 

 hypothetical substances that are contained in 

 the protective serum. 



When Ehrlich rendered animals immune 

 towards two proteid-like poisons of vegetable 

 origin, abrin from the jequirity-bean (Indian 

 licorice, Abrus prcecatorius] and ricin from the 

 castor-oil bean, tolerance of these respective 

 poisons seemed to consist in the fact that the 

 immune animals possessed in their serum a 

 kind of antidote, antitoxin, or " anti-substance" 

 (Antikbrper] as Ehrlich somewhat singularly 

 expressed it. These substances, which are 

 themselves proteid-like, were supposed to para- 

 lyze the proteid-like poison, or, as Behring 

 conceived, to neutralize it. The difference be- 

 tween active and passive immunization, be- 

 tween tissue immunity and antitoxin immu- 

 nity, consists, then, in the fact that in active 

 immunity the organism inoculated forms this 

 antitoxin but slowly and with the help of its 



21 



