THE PHENOMENA OF INFECTION 85 



of the failure of so many species of bacteria to 

 do harm to the higher animals. What has been 

 said about the production and utilization of 

 ferments by the bacterial cell is equally true 

 of the body cell. In fact, it is true of every liv- 

 ing cell. The body cell has its specific ferments, 

 and the bacterial cell being protein substance is 

 liable to be digested by the ferments elaborated 

 by the body cells. In these simple facts lies the 

 fundamental explanation of all forms of bac- 

 terial immunity, either natural or acquired. It 

 will be understood that I am here omitting all 

 reference to the elaboration of toxins and anti- 

 toxins. 



Ferments are intra- and extracellular. All 

 are formed within the cell, but some diffuse into 

 the medium; while others do not. In some in- 

 stances at least, cell permeation by the pabulum 

 is essential to the feeding of the cell. In other 

 cases the ferment accumulates on the surface 

 where digestion proceeds. In others the fer- 

 ment diffuses into the medium more or less 

 widely from the cell which produces it. Many 

 cells produce both intra- and extracellular fer- 

 ments, and these differ in function. 



I am not going into detail concerning cellular 

 ferments. Those of the bacterial cells are easi- 

 ly obtained and have been studied quite elabo- 



