136 PATHOGENIC BACTERIA. 



trations of antitoxin during a period of three months and 

 ten days he was unable to find demonstrable antitoxin in 

 the horse's blood. By a subsequent series of twelve 

 injections, during a period of two months and five days, 

 in which combined toxin and antitoxin were used, the 

 antitoxin was formed and a value of 320 units per cubic 

 centimeter was attained. These experiments furnish 

 altogether too little data to be conclusive, but indicate 

 that it is the toxin only that contributes to toxin forma- 

 tion, and that in the mixtures of toxin and antitoxin, 

 enough toxin remains unchanged to produce considerable 

 antitoxin. 



It would seem natural that if there is a true chemic 

 reaction between the toxin and antitoxin before injection 

 into the animal body, in exactly neutral mixtures the 

 antitoxin should be entirely destroyed by combining 

 with the toxin, which in its turn should have no re- 

 maining toxicity. Such a mixture should not confer 

 passive immunity because it no longer contains antitoxin, 

 nor yet stimulate active immunity because it contains no 

 toxin. The fact is, however, that after the injection of 

 a carefully made neutral mixture a high degree of im- 

 munity remains for some time. 



I think the final proof that the reaction is not a purely 

 chemic one is found in the calculations already given to 

 illustrate the paradox of hypersensitivity. In this condi- 

 tion we find animals with easily demonstrable amounts 

 of antitoxin in the blood dying from ordinary toxin 

 doses, no reaction between the toxin and antitoxin taking 

 place. It might be urged that for the occurrence of this 

 reaction some combining substance is necessary, but it 

 must be remembered that in the test-tube experiments 

 no such substance seemed to be necessary. 



3. Theory of a Combining Ferment. — As in the coagu- 

 lation of the blood and inflammatory exudates the union 

 of the fibrin factors and ferments will not take place 

 except in the presence of certain salts, so it may be true 

 that the toxin is unable to combine with the antitoxin 



