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 
