IMMUNITY 61 



protection against typhoid fever has attained marked 

 success. 



Phagocytosis. — In 1884 Metchnikoff ^ estabUshed 

 his celebrated theory of phagocytosis as an explana- 

 tion of immunity. Metchnikoff believes that the 

 wandering cells of the animal organism — leucocytes 

 — possess the power of digesting or rendering inert the 

 bacteria which they encounter in the tissues; and 

 that the susceptibihty to or immunity from infection 

 in an individual is decided by a conflict between the 

 invading bacteria on one side and the leucocytes 

 and tissues on the other, the outcome depending upon 

 the vigor of the invaders or upon the provisions for 

 defense set up by the leucocytes and tissues. 



When the vigor of the body cells is sufficient to 

 destroy the bacteria, the tissues are victorious, but 

 when the toxins produced by the bacteria are strong 

 enough to arrest the activity of the phagocytes, an 

 infection follows. 



"... the migration of the phagocytes through 

 the vessel wall into the cavities and tissues is one 

 of the principal means of defense possessed by an 

 animal. As soon as the infective agents have pene- 

 trated the body, a whole army of white corpuscles 

 (leucocytes) proceeds toward the menaced spot, 



' The successor of Pasteur in the Pasteur Institute, Paris. 



