IMMUNITY 203 



a fine capillary tube containing some peptone solution be 

 introduced into a suspension of bacilli, e.g. B. fluorescens 

 liquefaciens, under a cover- glass, and watched microscopi- 

 cally, the bacilli will be attracted to the tube and soon 

 invade its lumen. If, however, a weak acid be substituted 

 for the peptone water, the bacilli will be repelled. The 

 process by which the bacteria are ingested by the leucocytes 

 can be similarly watched. The leucocytes which act in 

 this manner are termed phagocytes, and they are of two 

 classes the macrophages, the large mononuclear leuco- 

 cytes, and the smaller microphages, or polymorphonuclear 

 leucocytes. Certain of the tissue cells and endothelial 

 cells also possess phagocytic properties. The importance 

 of phagocytosis is also shown by the fact that, while in 

 ordinary susceptible rabbits infection with anthrax is 

 followed by a feeble phagocytosis and the animals succumb, 

 in rabbits vaccinated against anthrax phagocytosis is very 

 active. Moreover, in an animal refractory to anthrax, 

 such as the frog, anthrax bacilli grow and multiply if they 

 be enclosed in paper or collodion sacs, so as to prevent the 

 access of the phagocytes. 



Phagocytosis, in vitro, and probably also in the normal 

 body, is extraordinarily active, so that it might be expected 

 always to be sufficient to deal with any number of bacteria 

 that might be introduced. If, however, the bacteria be 

 virulent, negative chemotaxis will occur. Moreover, the 

 presence of substances which render the bacteria phago- 

 cytosable, " opsonins," is necessary, and it seems likely 

 that the amount of opsonin becomes diminished in infection 

 (see p. 211). 



Metchnikoff admits that the destruction of bacteria in 

 phagocytosis is brought about by chemical bacteriolytic 

 substances, which he terms " cytases," and which he 

 regards as being derived from the leucocytes, and as 

 identical with the alexins. He believes that there are two 



