212 MANUAL OY BACTERIOLOGY. 



munity, " on the other hand, is that which results in some 

 cases : 



(i) From a spontaneous attack of an infectious disease; 

 (2) from an attack excited artificially through inoculation with 

 small doses of virulent cultures; (3) from the administra- 

 tion of large doses of attenuated cultures; (4) from the 

 injection of bacterial products (toxins) freed from the bacteria 

 themselves. Pasteur's methods of protective inoculation for 

 anthrax and other diseases, and Haffkine's injections for bu- 

 bonic plague, the gradual injection of cultures of diphtheria or 

 of tetanus bacilli into animals, produce active immunity. 

 Active immunity is usually more enduring than passive immu- 

 nity. But passive immunity, resulting, as it does, from the 

 direct introduction of antitoxin, is brought about more quickly 

 than active immunity. 



THEORIES OF 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 cer- 

 tain cells of the body have the power of surrounding and in- 

 gesting foreign substances. The cells in question are chiefly 

 polynuclear leukocytes, but to some extent other leukocytes 

 and endothelial and other cells are also concerned. The poly- 

 nuclear 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. Par- 

 ticles 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- 



*Greek, ^ayeiv, to eat; kijtos, a cell. 



