IMMUNITY. 213 



pigment. The presumption is that phagocytic cells serve to 

 remove irritating and foreign bodies and to destroy them. 

 Metchnikoff showed that phagocytes also absorb bits of degen- 

 erating or useless tissue. Such particles disintegrate, and they 

 are digested and become a part of the protoplasm of the 

 phagocytes. This process is seen when the tail of the tadpole 

 shortens. The superfluous part is absorbed, at least in part, 

 by phagocytic leukocytes. Metchnikoff' s earlier observations 

 were made largely on the invertebrates, whose transparent 

 bodies may be studied while living. One illustration was 

 furnished by a small crustacean (Daphnia or water-flea,) 

 which was often infected with a fungus. Some infected in- 

 dividuals died, others recovered. 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 phago- 

 cytes, 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 leukocytes. 

 Metchnikoff* contends that this function of leukocytes and 

 other phagocytic cells constitutes the principal defense of the 

 body against bacteria. 



Other investigators also have seen bacteria inclosed 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 inclosed 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 inclosed within leukocytes, it is quite evident 

 from their appearance that the bacteria retain their full vigor 

 after being ingested. 



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

 1893. 



