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 viruleift culture of one of these 

 microorganisms, 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 microorganism 

 when subsequently introduced or upon its toxic products. 



But there is another explanation which, although it may appear 

 a priori 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 microorganism. 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 septicaBmia 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- 



