504 APPLICATION OF METHODS OF BACTERIOLOGY 



of a poison of remarkable toxicity that is accountable for 

 the constitutional symptoms and pathological lesions by 

 which the disease is characterized. If by appropriate 

 methods this poison (toxin) be separated from the bacteria 

 by which it was formed, it is capable, when injected into 

 susceptible animals, of causing death and practically all the 

 lesions that accompany the disease when due to the invasion 

 of the living bacteria. If, on the contrary, the dose of poison 

 be so adjusted as to cause only temporary inconvenience 

 and not endanger life, and this dose be injected repeatedly, 

 gradually increasing in size as the animal is able to bear 

 it, after a while a marked tolerance is established, so that 

 the animal may be given many times the amount of the 

 toxin that would otherwise prove fatal i. e., many times 

 the lethal dose for an animal that had not acquired such a 

 tolerance. 



If blood be now drawn from the animal that has become 

 habituated, so to speak, to the diphtheria toxin, and the 

 serum collected from it, we discover several important 

 facts, viz.: 



That this serum when mixed with the previously deter- 

 mined lethal dose of the toxin in a test-tube will either 

 neutralize its toxicity or greatly reduce it, according to the 

 amount of serum used. 



That if we inject into an animal the determined fatal 

 dose of the toxin, and immediately afterward inject a quan- 

 tity of the .serum, either the animal will not die or the death 

 will be more or less delayed, according to the amount of 

 serum employed. 



That if a susceptible animal be inoculated with a living 

 culture of virulent bacterium diphtheriae, its life may be 

 saved, or its death postponed, by the subsequent injection 



