84 STUDIES IN IMMUNITY. 



tible to granular transformation in vaccinated animals even outside 

 the cells may show the same change in normal animals. But in the 

 latter case the transformation occurs only in those places where the 

 bactericidal substance is most concentrated, that is to say, within 

 the phagocyte." We may add that if we inject B. coli into a 

 guinea-pig with preventive serum we find that granular transforma- 

 tion takes place within the leucocytes even when the bacteria in the 

 surrounding fluid are unaffected. 



These facts indicate very clearly the superiority in bactericidal 

 power of the white corpuscles over the body fluids, as is shown by 

 a production of granules. They show, moreover, that the bacteri- 

 cidal power in an exudate owes its origin entirely to the leucocytes 

 in the fluid. We must consider why this essential function of 

 leucocytes in the elaboration of bactericidal substance has been 

 so much doubted and is still denied by many observers, and why 

 the activity of the body fluids in destroying bacteria has seemed 

 in certain cases superior to the cell activity. It is due simply to 

 the following fact : in well-immunized animals a very small dose of 

 bactericidal substance is sufficient to cause a granular change in 

 a large number of vibrios.* A very few leucocytes endow the 

 exudate with a bactericidal power which can produce the meta- 

 morphosis of a surprising number of bacteria. 



On the other hand, a large supply of leucocytes is necessary to 

 effect the phagocytosis of a moderate number of bacteria. When 

 vibrios are injected into the peritoneal cavity the few leucocytes 

 present suffice to give the peritoneal fluid a very distinct bactericidal 

 activity, but as they are few in number they cannot take up many 

 organisms. For this reason the attention of the observer is imme- 

 diately monopolized by the extracellular bactericidal activity, so 

 that he overlooks the essential importance of the cells present in the 

 fluid. Later on the leucocytes come .up in large numbers and 

 phagocyte the intact or altered vibrios completely and then it is 

 quite impossible even on the most superficial examination to over- 

 look the importance of their protective function. In short, extra- 

 cellular transformation of vibrios in the peritoneal cavity is due to 



* Two or three platinum loops of the serum of one of our vaccinated guinea- 

 pigs, when mixed with a drop of emulsion of vibrios, cause a generalized meta- 

 morphosis, and yet our serum is not, as a rule, as active as Pfeiffer's. 



