244 ACQUIRED IMMUNITY 



neutralize and render harmless the toxins much as an acid 

 neutralizes a base and so enable the phagocytes to overcome 

 their enemies. The supporters of this hypothesis point to the 

 suggestive facts that, within limits, toxins are rendered harmless 

 to an animal if the corresponding antitoxins are first injected in 

 sufficient quantities, and that, if mixed with the antitoxins before 

 injection, they are neutralized more thoroughly, and therefore are 

 less poisonous than when injected simultaneously but separately, 

 and, further, that when mixed outside the host a definite amount of 

 antitoxin will render inert a definite amount of toxin. But a 

 number of additional facts indicate that the neutralization theory 

 is untenable. If a fatal dose of toxin be rendered harmless by a 

 minimum quantity of antitoxin, and then another lethal dose 

 of toxin be added to the mixture, the latter should become lethal 

 once more. But it does not, it merely causes a very slight illness. 

 It needs the addition of quite a number of lethal doses to make 

 the mixture deadly. Again a mixture of toxin and antitoxin may 

 be made which is harmless for one animal (e.g. mouse) but 

 poisonous to another (e.g. guinea pig), a fact which proves that the 

 reaction that results in immunity is, not merely a chemical, but a 

 vital one. Yet again, if a mixture of diphtheria toxin and antitoxin 

 which is harmless to a mouse be made, it is rendered poisonous by 

 heating it to the boiling point of water. Similarly if the mixture 

 be placed in a porcelain filter the toxin passes through, but the anti- 

 toxin remains, a fact which is decisive that the interaction between 

 the two substances is not of the nature of a chemical combination. 

 409. Both toxins and antitoxins are substances of highly 

 complex chemical composition ; each toxin is * neutralized ' only by 

 the corresponding antitoxin ; the power of producing its particular 

 toxin has been evolved by every species of microbe ; but antitoxins 

 as shown by the acquirement of immunity may be produced by 

 races which have had no previous experience of the corresponding 

 diseases ; antitoxins have been found in the blood of animals a few 

 minutes after the injection of the corresponding toxins. Therefore 

 the hypothesis of the chemical neutralization of toxins by antitoxins 

 elaborated by the individual attacked involves the assumption that 

 the animal body is a species of magic bottle which instantly 

 produces, under conditions never perhaps experienced by the race, 

 elaborate substances exactly suited to the needs of the moment. 

 It is, in fact, a hypothesis of immunization by miracle. Equally 

 founded on an appeal to the supernatural is the hypothesis, which 

 is or was popular, that the microbes after producing toxins which 



