82 STUDIES IN IMMUNITY. 



studied. Furthermore, attempts have been made to gain an insight 

 into tin' mechanism of the immunity conferred by serum, and to 

 determine the relation between this kind of immunity (passive 

 immunity) and the immunity caused by repeated injections of 

 bacteria (active immunity). In these discussions the question has 

 repeatedly recurred as to the part which is played in animal pro- 

 tection by the body fluids on the one hand and by the living cells 

 on the other. 



I. The Localization of the Bactericidal Substance 



During Life. 



If cholera vibrios are placed in the serum of an animal immunized 

 against these bacteria they are destroyed at least partially. After 

 a certain time cultures from this serum on artificial media are either 

 negative or show very few colonies. We have already given the 

 reasons that have led us to conclude that the bactericidal substance 

 present in serum comes from leucocytes. We were forced to the 

 conclusion that during life the bactericidal substance is present in 

 leucocytes and that when the white blood cells are removed from 

 the blood vessels they liberate into the surrounding serum those 

 bactericidal substances which they normally retain.* We based 

 our conclusions on the comparative study of the bactericidal 

 property of the serum of a vaccinated animal with the edema 

 fluid from the same animal, and by a comparison between the 

 destructive property of the serum from whole blood and the serum 

 from blood previously deprived within the body of part of its 

 leucocytes. 



The means of determining the existence of bactericidal power 

 and of estimating its intensity was simply to inoculate culture media 

 (gelatin) with organisms that had been allowed to remain in contact 

 with the body fluids mentioned for variable lengths of time. We 

 have also considered another reaction that has been given to inves- 

 tigators through the observations of Pfeiffer. 



Vibrios and other bacteria may show the harmful influences of a 

 bactericidal substance, not only by losing more or less completely 

 their power to develop on such artificial media as gelatin, but also 

 by the morphological change they show when affected by this 



* See page 24. 



