258 SUSCEPTIBILITY AND IMMUNITY. 



pathogenic microorganisms that it is difficult to believe that they 

 have all been picked up by . a voracious phagocyte, which has 

 stuffed itself to repletion, while numerous other leucocytes from 

 the same source and in the same microscopic field of view have 

 failed to capture a single bacillus or micrococcus. Moreover, the 

 staining of the parasitic invaders, and the characteristic arrange- 

 ment of the "gonococcus" in stained preparations of gonorrhceal 

 pus, indicate that their vitality has not been destroyed in the interior 

 of the leucocytes or pus cells, and we can scarcely doubt that the 

 large number found in certain cells is due to multiplication in situ 

 rather than to an unusual activity of these particular cells. But in 

 certain infectious diseases, and especially in anthrax, the bacilli in- 

 cluded within the leucocytes often give evidence of degenerative 

 changes, which would support the view that they are destroyed by 

 the leucocytes, unless these changes occurred before they were picked 

 up, as is maintained by Nuttall and others. We cannot consider 

 this question as definitely settled. 



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 v/ay, 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, 



