
1905.| Substance in Serum which influences Phagocytosis. 507 
It was found, however, that cases of protection resulting from the use of 
immune serum occurred where no such bacteriolytic action could be demon- 
strated; the plague bacillus and the streptococcus may be mentioned as 
examples. Admitting that the phagocyte plays a part in the protection 
against these infections, the question must still be considered whether the 
immune serum has acted on the injected microbes or on the phagocytes, 
Metchnikoff maintains that the serum stimulates the leucocyte to its 
activity, whereas many workers, who are’ quite prepared to admit the 
important part which the phagocyte plays in the process, hold that the 
immune serum acts chiefly on the micro-organism. 
Metchnikoff's view, however, is not opposed to the idea of the immune 
substance, or “ substance sensibilisatrice,” becoming fixed on to the cocci. He 
admits that this occurs, and that the micro-organisms thus sensitised, though 
they maintain their vitality and virulence, become more readily the prey of 
the leucocyte, whose activity is increased by the stimulating action of the 
“substance sensibilisatrice.” In the animal body, under normal conditions, 
bacteriolysis of the microbe occurs within the phagocyte (Bordet and 
Levaditi). In experiments in vitro, or in the animal body where 
phagolysis has occurred, free cytase or complement being present, bac- 
teriolysis may occur both outside and inside the phagocyte. 
It may be of use here to make a brief reference to a few of the investiga- 
tions carried out by the followers of Metchnikoff with reference to the 
influence exerted by the serum, on the one hand on the phagocytes, on the 
other hand on the microbes. The papers selected to illustrate the subject 
are by Bordet, Savtschenko, and Levaditi, and their views are referred to only 
in so far as they touch on these points. 
Bordet (1895 and 1897) holds that the specific serum contains a thermo- 
stable substance, “ sensibilisatrice,’ which acts on the micro-organisms and 
prepares them for the thermolabile alexin, or proteolytic ferment, which acts 
as the solvent. He compares the action of the immune serum to that of a 
mordant. In certain cases, however, such as in streptococcus infection, the 
bactericidal action is slight, and in such cases he attributes to the immune 
serum a stimulating action on the leucocytes. The leucocytes and other cells 
cam perceive the presence of a preventive serum, and under its stimulus they 
are capable of reacting by movement. They manifest towards the immune 
serum a pronounced positive chemiotaxis. The activity of the leucocytes 
in the presence of such serum can be observed in vitro. 
The bactericidal substance is not uniformly distributed through the plasma, 
but during life is confined within the leucocytes. 
Savtschenko and Melkich (1901), from their study of the processes observed 
