178 MANUAL OF BACTERIOLOGY. 



bacterial products (toxins) freed from the bacteria themselves. 

 Pasteur's methods of protective inoculation for anthrax and 

 other diseases, and Haffkine's injections for bubonic plague, 

 produce active immunity. Active immunity is usually more 

 enduring than passive immunity. But passive immunity, result- 

 ing, as it does, from the direct introduction of antitoxin, is 

 brought about more quickly than active immunity. 



Theories or Immunity. 



Phagocytosis.* — Metchnikoff described under the name 

 "phagocytosis" immunity and recovery from bacterial inva- 

 sion. 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 leukocytes, but to some extent other leukocytes and 

 endothelial and other cells are also concerned. The polynu- 

 clear leukocytes are the cells which destroy bacteria, and 

 Metchnikoff now calls these microphages; other phagocytes he 

 calls macrophages. There are many examples of phagocytosis 

 which have been observed. The phagocytes of the lungs con- 

 stantly take up small bits of carbon inhaled with the air. 

 Particles of carmine injected into the tissues will later be found 

 within phagocytes. 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 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 earher observations were 

 made largely on the invertebrates, whose transparent bodies may 



* Greek, faytiT, to eat; kOtoc, a. cell. 



