IMMUNITY. 327 



of this toxin solution is nullified by 0.0500. 

 of a serum, on simultaneous injection, ice. of 

 the serum evidently protects 100,000 grams of 

 animal against the minimal dose, and the im- 

 munizing value is hence, according to Roux, 

 100,000. With very potent serum, infini- 

 tesimal quantities are sufficient to produce 

 immunity ; in diphtheria about ywVo F f 

 the body weight and in tetanus about one-tril- 

 lionth can protect the animal. 



The Nature of Antitoxin. 



These facts must not cause us to mistake 

 the action of antitoxin for a neutralization, a 

 direct nullifying of the poison by the antidote 

 in the sense of a molecular interchange siich 

 as occurs between acid and alkali, although 

 such a comparison has been used by Behring. 

 The action of antitoxin, so far as at present 

 known, is simply that of a cell stimulus. We 

 are even able to prove directly that the protec- 

 tive serum, or the supposititious antidote in it, 

 does not actually destroy or neutralize the 

 poison used in immunizing or in establishing 

 a tolerance of poison, but that both bodies can 

 exist side by side. 



Buchner prepared a mixture of tetanus toxin 

 and serum which was neutral in the Behring- 



