IMMUNITY. 183 



ingested and destroyed by the leucocytes 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 bacilli were 

 ingested by the frog's leucocytes. MetchnikofF contended 

 that this function of leucocytes and other phagocytic cells 

 constituted the principal defence of the body against bac- 

 teria. 



Other investigators also have seen bacteria enclosed 

 within the bodies of leucocytes. It has been urged by some 

 that the bacteria are already dead when the leucocytes de- 

 vour them. In some cases, as with the gonococcus which is 

 commonly found enclosed within leucocytes, it is possible 

 that the bacteria retain their full vigor after being ingested. 



It is well known that a suppurating part contains large 

 numbers of leucocytes, and one of the most characteristic 

 events in the inflammatory process is the migration of 

 leucocytes to the point of irritation. This indicates a posi- 

 tive chemotaxis for leucocytes on the part of substances in 

 the inflamed area. Metchnikoff believed that the function 

 of these leucocytes is to destroy the bacteria and to arrest 

 their further progress. On this theory bacteria have often 

 been likened to an invading army and the leucocytes or 

 phagocytes to a force designed to repel their attacks. 



It is certain that in some infectious diseases the number 

 of leucocytes, chiefly of the polynuclear neutrophilic variety, 

 in the circulating blood is increased (leucocytosis). This is 

 the case usually in lobar pneumonia and acute suppurative 

 infections. In other infectious diseases there is no leucocy- 

 tosis ; for example, tuberculosis, typhoid fever and malaria. 

 It is interesting to observe that in trichinosis, and more 

 rarely in infection with other animal parasites, the eosino- 



1 Metchnikoff, " Comparative Pathology of Inflammation," trans., 

 Starling, 1893. 



