1 26 Immunity 



These devouring cells Metschnikoff called phagocytes, and 

 of them he recognized two classes, the microphages , which 

 are white blood-corpuscles, and the macrop hages, which are 

 larger cells derived from the endothelial and other tissues. 

 Metschnikoff, his associates, and his pupils soon collected 

 evidence sufficient to show that phagocytosis, if not the 

 chief factor in defending the body from infectious organisms, 

 is at least an important one. Many of the most interesting 

 facts are described in Metschnikoff 's books, "Etudes sur Tin 



Fig. 1 8. Phagocytosis; the omentum immediately after injection of 

 typhoid bacilli into a rabbit. Meshwork showing a macrophage, inter- 

 mediate forms and a trailer, all containing intact bacilli (Buxton and 

 Torry). 



flammation" and "Immunite dans les Maladies Infectieuses," 

 which every interested student of the subject should read. 



These studies show that in nearly all cases in which 

 animals are naturally immune against infection, the leu- 

 kocytes are active in their phagocytic behavior toward 

 them; that in acquired immunity, the leukocytes pre- 

 viously inactive, become active toward them; that the 

 enclosure of bacteria within the cells sometimes results in 

 the death of the cells, sometimes in the death of the bacteria ; 

 that phagocytosis is much more active in diseases in which 

 the bacteria have limited toxicogenic powers, and in which 

 they probably exert a positively chemotactic influence upon 

 the cells, than in cases in which the bacteria are strongly 

 toxicogenic and probably exert an injurious and negatively 

 chemotactic influence upon them, and that when the toxico- 

 genic power of the bacteria is great, many of the phago- 

 cytes are killed and dissolved phagolysis. Study of the 

 primitive forms of animal life shows that amebae constantly 



