INFECTION ' 255 



one may produce either prompt death or only slight con- 

 stitutional reaction. In the latter event repeating the 

 injection of a non-fatal dose may have no apparent effect 

 upon the animal. In such a case the animal has acquired, 

 loosely speaking, a tolerance to the poison and this tolerance 

 is due to a newly formed, antidotal substance now circulating 

 in the blood of the tolerant or immune animal. For example: 

 If a measured quantity of the toxin under consideration be 

 mixed in test-tubes with varying amounts of the serum of 

 the tolerant animal and each of these mixtures be injected 

 into fresh, nornaal animals of the same species, it will be 

 seen that in some instances the toxicity of the poison is 

 only lessened, while in others it may be completely neutral- 

 ized; in other words, we have demonstrated by such an 

 experiment the presence in the blood of an antidote, and 

 "antitoxin" as it is called. This antidote is specific, that 

 is, it can neutralize only the poison used in the experiment; 

 it is inactive when used against other toxins. 



This union between toxin and its antidote is conceived to 

 occur according to the laws governing ordinary chemical 

 reactions, i. e., there is a definite numerical relationship; 

 a certain fixed quantity of toxin being neutralized by a 

 certain fixed amount of antitoxin, variations in either factor 

 resulting in failure to accurately neutralize. The union 

 between the two factors is made possible, according to 

 Ehrlich's conception, through the possession by the toxin 

 molecule and by the antitoxin molecule of constituents 

 having the combining function, "haptophore" side chains, 

 as he calls them. In addition the toxin molecule possesses 

 another constitutent having the poisonous destructive func- 

 tion, the "toxiphoric," side chains, while the antidotal or 

 antitoxic molecule possesses a constitutent having the neu- 



