SUSCEPTIBILITY AND IMMUNITY. 



subsequently introduced into the body of the immune animal. We 

 <3annot understand how toxic substances introduced in the first in- 

 stance can neutralize substances of the same kind introduced at a 

 later date. There is something in the blood of the rat which, accord- 

 ing to Behring, neutralizes the toxic substances present in a filtered 

 culture of the tetanus bacillus ; but whatever this substance may be, 

 it is evidently different from the toxic substance which it destroys, 

 and there is nothing in chemistry to justify the supposition last 

 made. Is it, then, by destroying the pathogenic microorganism 

 that these inoculated and retained toxic products preserve the animal 

 from future infection ? Opposed to this supposition is the fact that 

 the blood of an animal made immune in this way, when removed 

 from the body, does not prove to have increased germicidal power as 

 compared with that of a susceptible animal of the same species. 

 Again, these same toxic substances in cultures of the anthrax bacillus, 

 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 

 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 

 i n the bod i i <>f I lie 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- 

 peri ni nu 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- 



