DIPHTHERIA BACILLUS. 357 



increase in the acidity or alkalinity of the bouillon in 

 which the diphtheria bacilli are producing their toxin 

 will prevent further production, it is easy to imagine that 

 many changes in the throat secretion or in its mucous 

 membrane may prevent the development of the bacil- 

 lus or of the production by the bacillus of its toxin, 

 and, therefore, of its disease-producing power. But, 

 as the result of animal experiments, it is now known 

 that an artificial immunity against diphtheria can be 

 produced, at least for a considerable length of time, 

 by the development of substances directly antidotal to 

 the diphtheria toxin. By the inoculation of virulent or 

 somewhat attenuated cultures or of diphtheria toxin, 

 Fraenkel, Behring, Wernicke, Aronson, Roux, and 

 since then many others, have succeeded in immunizing 

 animals; but the most important and valuable results 

 are those which have been obtained by Behring, in 

 conjunction with others, who showed that the blood 

 of immune animals contains a substance which neutial- 

 izes the diphtheria toxin. The blood-serum of persons 

 who have recovered from diphtheria has been found 

 also to possess this protective property, which it acquires 

 about a week after the beginning of the disease, and loses 

 again in a few weeks or months. Moreover, the blood- 

 serum of many individuals, usually adults, who have 

 never had diphtheria often has a slight general anti- 

 toxic property. 



Antitoxic Serum. The knowledge derived from these 

 remarkable investigations into the protective powers of 

 the blood-serum of immunized animals has been em- 

 ployed with the most brilliant results for the prevention 

 and early treatment of diphtheria in man. The dis- 

 covery of the method of the production of antitoxic 



