IMMUNITY AND SUSCEPTIBILITY. 79 



When this point is, however, safely passed, the increase 

 in dosage can be very rapid, yet without signs of poison- 

 ing, seemingly because the drug is no longer simply tol- 

 erated, but tolerated and simultaneously neutralized. By 

 experimentation Ehrlich has shown that during the 

 period of simple tolerance the blood of the animal is 

 unaltered, but that as soon as the tolerance becomes 

 unlimited the blood contains a new substance, capable 

 not only of protecting the animal by which it is pro- 

 duced, but also other animals into whose blood it is in- 

 troduced. In the ricin experiments this substance was 

 described as antiricin ; in the experiments with abrin, as 

 antiabrin. 



These investigations of Ehrlich with the poisons of 

 higher plants succeeded, but threw much light upon, the 

 extraordinary work of Behring, Wernicke, and Kitasato, 

 who experimented with the toxins of diphtheria and 

 tetanus, and showed that in the blood of animals accus- 

 tomed to these poisons, new substances antitoxins, found 

 by Brieger to be proteid in nature were produced. 



The antitoxic theory of immunity, being, in the cases 

 cited at least, a fact capable of demonstration, has estab- 

 lished itself at present as the most important hypothesis. 

 According to it, acquired immunity, at least, depends upon 

 the development in the blood of a neutralizing substance 

 probably related to the nucleins. 



It is of prime importance to remember that the anti- 

 toxin is an entirely new substance which does not occur 

 in the blood of normal animals, even when they possess 

 a high degree of natural immunity, except in rare in- 

 stances, and then only in minute amounts not propor- 

 tional to the degree of immunity. Calmette has called 

 special attention to this fact, and points out that while 

 fowls and tortoises resist abrin, their blood contains no 

 anti-abrin; Vaillard has shown that, although the fowl 

 resists tetanus, its blood contains no protective substance 

 destructive to tetanus-toxin. Calmette finds that the 

 blood of the ichneumon and hedgehog, which are im- 



