SUSCEPTIBILITY AND IMMUNITY. 255 



pears to play an important part in enabling immune animals to resist 

 invasion by pathogenic bacteria. 



Going back to the demonstrated fact that susceptible animals may 

 be made immune by inoculating them with the toxic products pro- 

 duced during the growth of certain pathogenic bacteria, we may 

 suppose either that immunity results from the continued presence of 

 these toxic products in the body of the inoculated animal, or from a 

 tolerance acquired at the time of the inoculation and subsequently 

 retained by transmission from cell to cell, as heretofore suggested. 

 Under the first hypothesis retention theory immunity may be ex- 

 plained as due to a continued tolerance on the part of the cellular ele- 

 ments of the body to the toxic substances introduced and retained ; 

 or to the effect of these retained toxic products in destroying the 

 pathogenic bacteria, or in neutralizing their products when these are 

 subsequently introduced into the body of the immune animal. "We 

 cannot 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 



