374 BACTERIOLOGY. 



products accumulate in the body of the host 

 and are injurious to the bacteria forming them 

 in the way that lactic acid is injurious to the 

 lactic acid bacteria, or alcohol to the yeast cell, 

 is in a restricted sense correct, and agrees 

 with what we know of the formation of anti- 

 substances, but the hypothesis itself has no 

 special significance. 



The distinction between natural immunity 

 and artificial immunity can no longer be main- 

 tained. In both, the wandering cells may be 

 able to take up parasites. Even in naturally 

 immune animals, anti-substances may at times 

 be present. Abel has found sometimes present 

 in the blood of healthy men an anti-substance 

 that protects against the diphtheria toxin; 

 Stern has found one protecting against the 

 germ of typhoid fever, and MetschnikofT one 

 against cholera, and on the other hand, as al- 

 ready mentioned, animals that are specifically 

 immunized against these diseases, may not pos- 

 sess any anti-substances in their serum. 



The conditions of hyper-immunity, natural 

 immunity and artificial immunity are all resi- 

 dent alike in the cells and tissues of the body. 

 The wandering cells of an animal may be en- 

 dowed from the outset with special voracity or 

 they may acquire this property, for the wander- 

 ing cells, of course, are both directly and indi- 



