IMMUNITY. 337 



has been considered by Beliring and Ehrlich 

 as " passive." Now in the passive immuniza- 

 tion that is effected in the case of diphtheria, 

 the animal still remains immnne after the anti- 

 substances have disappeared from the blood, 

 and experiments made outside of the body show 

 that the blood has been robbed of all protective 

 power. The explanation of this is to be found 

 in the fact that the poison tolerance was effected 

 by means of old cultures which contained, 

 besides the poison, those bacterial cell sub- 

 stances which have already been shown to be 

 the most important factors in active immuni- 

 zation (p. 311). 



If rabbits are inoculated with toxin-free te- 

 tanus spores together with lactic acid, the ani- 

 mals become immune, but their serum imparts 

 to other animals no protection against tetanus 

 (Roux and Vaillard). Immunity and poison 

 tolerance are evidently, in this instance, not 

 associated with the presence of antitoxin in 

 the blood. Behring has found that it is pos- 

 sible by means of too frequent or too large in- 

 jections of poison to immunize animals so 

 highly or more correctly to administer to 

 them so much poison that they exhibit symp- 

 toms of poisoning and become hyper-sensitive 

 to the toxin, in spite of the fact that their serum 

 displays the most powerful antitoxic action. 



22 



