632 THE POPULAR SCIENCE MONTHLY. 



presence of these toxic products in the body of the inoculated ani- 

 mal, or to a tolerance acquired at the time of the inoculation and 

 subsequently retained, by transmission from cell to cell, as here- 

 tofore suggested. Under the first hypothesis retention theory 

 immunity may be explained as due to a continued tolerance on 

 the part of the cellular elements 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 can not understand how toxic 

 substances introduced in the first instance can neutralize sub- 

 stances of the same kind introduced at a later date. There is 

 something in the blood of the rat which, according to Behring, 

 neutralizes the toxic substances present in a filtered culture of the 

 tetanus bacillus; but whatever this substance may be, it is evi- 

 dently 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 micro-organism, 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 re- 

 moved 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 micro-organisms, the toxic sub- 

 stances present do not prevent the rapid development of the bacil- 

 lus ; indeed, instead of proving a germicide they favor its develop- 

 ment, 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 it is evident that acquired 

 immunity does not result from the direct action of the products 

 of bacterial growth, introduced and retained in the body of the 

 immune animal, upon the pathogenic micro-organism when sub- 

 sequently introduced, or upon its toxic products. 



But there is another explanation which, although it may ap- 

 pear a priori to be quite improbable, has the support of recent 

 experimental evidence. This is the supposition that some sub- 

 stance is formed in the body of the immune animal which neutral- 

 izes the toxic products of the pathogenic micro-organism. How the 

 presence of these toxic products in the first instance brings about 

 the formation of an "antitoxine" by which they are neutralized 

 is still a mystery ; but that such a substance is formed appears to 

 be proved by the recent experiments of Ogata, Behring and Kita- 



