18 STUDIES IN IMMUNITY. 



these conditions that an increase in virulence occurs, due apparently 

 to selection by the phagocytes.* Bacteria that develop in the tissues 

 of refractory animals have little attraction for leucocytes, which is 

 the proof of their pathogenic power. The experiments of Charrin 

 and Roger on the attenuation of Bacillus pyocyaneus are too open 

 to criticism to furnish any reasonable data for the doctrines of an 

 attentuation by body fluids. 



Since the intervention of phagocytes is regularly observed in 

 animals that defend themselves against invading organisms, the 

 phagocytic theory is fully capable of explaining the problem of 

 immunity. But although the fact that phagocytes are the usual 

 factors in immunity is undeniable, their importance in certain 

 special cases may be limited. These limitations have been found 

 particularly in infections caused by organisms belonging to the 

 group of vibrios; and it is particularly in regard to the cholera 

 vibrio that the discussion between the phagocytic doctrine and the 

 "humoral" theory has grown very active. There is, in fact, in 

 these special cases an evident correlation between the appearance 

 of a bactericidal power in serum and the beginning of the refractory 

 condition. This destructive property, though very slight in normal 

 animals, becomes very marked in vaccinated animals, and if it 

 should be shown in such animals that the destructive properties 

 of the serum are present during life, the intervention of phagocytes 

 would no longer appear necessary in dissipating the infection; this 

 point, however, is one that has never been demonstrated. The 

 researches of various observers, and particularly of Metchnikoff, 

 tend to convince us that the tissue fluids possess during life no 

 bactericidal property comparable to that shown by the serum in 

 vitro.j 



One of the principal objects of the present study is to deter- 

 mine whether these bactericidal substances are not present in cells 

 in the living body. It is to be determined whether it is the leuco- 

 cytes particularly that contain the bactericidal substances, whether 

 the elaboration of these substances is a special manifestation of 

 their protective activity, and whether this activity is brought about 



* See p. 4. 



t See Metchnikoff, Etudes sur I'immunite", 4 e memoire. Ann. de 1'Institut 

 Pasteur, 1891. 



