1 82 MANUAL OF BACTERIOLOGY. 



immunity. Active immunity is usually more enduring than 

 passive immunity. Passive immunity, established through 

 the direct introduction of antitoxins, may be brought about 

 more quickly than would be possible for an active immunity. 



THEORIES OF IMMUNITY. 



Phagocytosis. 1 Metchnikoff described under the name 

 " phagocytosis " a phenomenon which, he maintained, could 

 explain immunity and recovery from bacterial invasion. 

 This theory is based on the well-known fact that certain cells 

 of the body have the power of surrounding and ingesting 

 foreign substances. The cells in question are chiefly poly- 

 nuclear leucocytes, but to some extent other leucocytes, and 

 enclothelial and other cells. There are many examples of 

 this process. The leucocytes of the lungs constantly take up 

 small bits of carbon inhaled with the air. Particles of car- 

 mine injected into the tissues will later be found within 

 leucocytes. After a hemorrhage, phagocytic cells may be 

 found containing red blood-corpuscles or particles of blood 

 pigment. The presumption is that phagocytic cells serve to 

 remove irritating and foreign bodies to less sensitive parts. 

 Metchnikoff showed that phagocytes also absorb bits of de- 

 generating or useless tissue. Such particles disintegrate 

 and their identity is lost. 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 leucocytes. 

 MetchnikofFs observations were made largely on the in- 

 vertebrates, 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 individuals died, others recovered. 

 Metchnikoff found that the cells of the fungus might be 



1 Greek, (bayelv, to eat ; nvroq , a cell. 



