ACQUIRED IMMUNITY 119 



authorities antitoxins are supposed to be elaborated by the 

 infected person, by another set they are supposed to be 

 produced by the parasites. Many facts have been thought 

 to furnish evidence of the truth of the neutralization theory. 

 Some parasites, the diphtheria bacilli, for example, can be 

 cultivated outside the body in broth or other nutrient media, 

 which then becomes intensely poisonous from the presence of 

 toxins. A horse can be gradually habituated to having large 

 quantities of this poisoned broth injected into him. There- 

 after his serum possesses remarkable properties. It is no 

 longer poisonous, at any rate it is not poisonous if withdrawn 

 from the horse after a sufficient interval of time. On the 

 contrary, it possesses curative powers. If mixed with a given 

 proportion of the toxin it renders the latter harmless. If the 

 toxin and the antitoxin are injected simultaneously into 

 different parts of an animal, the same result follows, though 

 in this case somewhat larger doses of antitoxin are needed. 

 Even when injected into an animal already diseased, and in 

 which, therefore, the toxins are already present, it greatly 

 tends to promote recovery. On this latter discovery is 

 founded the celebrated serum treatment of diphtheria which 

 has saved so many lives. 



193. The doctrine of chemical neutralization has been very 

 fruitful from a practical point of view. The men who for- 

 mulated it and worked under its guidance are those who have 

 built up our present splendid system of serum therapy, and 

 who have thus robbed some death-dealing complaints of half 

 their terrors. But a working hypothesis, untrue in itself, may 

 occasionally lead to important practical results. There are ap- 

 parently insuperable objections to the theory of neutralization. 



194. Antitoxins have been detected in the blood of an 

 animal a few minutes after the injection of toxins. Evidently 

 they are specific substances, since immunity conferred by 

 them against one disease does not protect against any other. 

 It is improbable in the last degree that the animal body is a 

 species of magic bottle, instantly capable of producing at 

 need highly complex chemical substances, the antitoxins, 

 which exactly neutralize other equally complex substances, 

 the toxins, the right antitoxin at the right time ; or that 

 each toxin contains, or is capable of being converted into, 

 substances chemically antagonistic to itself, and to no other 

 toxin. If this happens it is a fact unique in nature. 

 Nothing else like it is known to occur. It is possible perhaps 

 to formulate an hypothesis which is at once more probable 

 and more in accordance with the rest of our experience. 



