IMMUNITY. 1 79 



be studied while living. One illustration was furnished by a 

 small crustacean {Daphnia or water-flea), which was often in- 

 fected with a fungus. Some infected individuals died, others re- 

 covered. Metchnikoff found that the cells of the fungus might 

 be ingested and destroyed by the leukocytes of the Daphnia. 

 He described the history of this disease as a contest between 

 the parasitic cells and the phagocytes, in which either might 

 succeed. Similarly, when anthrax bacilli were introduced 

 into frogs, which are immune from anthrax, the bacilh were 

 ingested by the frog's leukocytes. Metchnikoff* contends 

 that this function of leukocytes and other phagocytic cells 

 constitutes the principal defence of the body against bac- ^ 

 teria. 



Other investigators also have seen bacteria enclosed within 

 the bodies of leukocytes. It has been urged by some that 

 the bacteria are already dead when the leukocytes devour them, 

 but Metchnikoff showed that these enclosed bacteria are still 

 alive, for they produce disease when introduced into fresh ani- 

 mals; so they are apparently not injured before they are taken 

 up. In other cases, as with the gonococcus, which is com- 

 monly found enclosed within leukocytes, it is quite evident 

 from their appearance that the bacteria retain their full vigor 

 after being ingested. 



It is well known that a suppurating part contains large 

 numbers of leukocytes, and one of the most characteristic 

 events in the inflammatory process is the migration of leuko- 

 cytes to the point of irritation. This indicates a positive 

 chemotaxis for leukocytes on the part of substances in the in- 

 flamed area. Metchnikoff believes that the function of these 

 leukocytes is to destroy the bacteria and to arrest their further 

 progress. On this theory bacteria have often been likened to an 



* Metchnikoff. Comparative Pathology of Inflammation. Trans., Starling. 

 1893. 



