SUSCEPTIBILITY AND IMMUNITY. 259 
the tetanus bacillus, the diphtheria bacillus, etc., do not destroy the 
pathogenic germ after weeks or months of exposure. And when we 
inoculate a susceptible animal with a virulent culture of one of these 
microérganisms, the toxic substances present do not prevent the rapid 
development of the bacillus ; indeed, instead of proving a germicide, 
they favor its development, which is more abundant and rapid than 
when attenuated cultures containing less of the toxic material are 
used for the inoculation. In view of these facts we are unable to 
adopt the view that acquired immunity results from the direct action 
of the products of bacterial growth, introduced and retained in the 
body of the immune animal, upon the pathogenic microédrganism 
when subsequently introduced or upon its toxic products. 
But there is another explanation which, although it may appear 
a priort to be quite improbable, has the support of recent experimen- 
tal evidence. This is the supposition that some substance is formed 
in the body of the immune animal which neutralizes the toxic 
products of the pathogenic microdrgantsm. How the presence of 
these toxic products in the first. instance brings about the formation 
of an “antitoxin” by which they are neutralized is still a mystery; 
but that such a substance is formed appears to be proved by the ex- 
periments of Ogata, Behring and Kitasato, Tizzoni and Cattani, G. 
and F. Klemperer, and others. 
Ogata and Jasuhara, in a series of experiments made in the Hy- 
gienic Institute at Tokio (1890), discovered the important fact that 
the blood of an animal immune against anthrax contains some sub- 
stance which neutralizes the toxic products of the anthrax bacillus. 
When cultures were made in the blood of dogs, frogs, or of white 
rats, which animals have a natural immunity against anthrax, they 
were found not to kill mice inoculated with them. Further experi- 
ments showed that mice inoculated with virulent anthrax cultures 
did not succumb to anthrax septicemia if they received at the same 
time a subcutaneous injection of a small quantity of the blood of an 
immune animal. So small a dose as one drop of frog’s blood or one- 
half drop of dog’s blood proved to be sufficient to protect a mouse 
from the fatal effect of an anthrax inoculation. And the protective 
inoculation was effective when made as long as seventy-two hours 
before or five hours after infection with an anthrax culture. Fur- 
ther, it was found that mice which had survived anthrax infection as 
a result of this treatment were immune at a later date (after several 
weeks) when inoculated with a virulent culture of the anthrax 
bacillus. 
Behring and Kitasato have obtained similar results in their ex- 
periments upon tetanus and diphtheria, and have shown that the 
blood of an immune animal, added to virulent cultures before in- 
