Phagocytosis in Immunity 103 



and digested, and that the behavior of the cells toward the 

 bacteria afforded an explanation of the mechanism by which 

 recovery from the infectious diseases takes place. According 

 to the original interpretation upon which this, the "theory 

 of phagocytosis" was founded, recovery in many if not all 

 of the infectious diseases depends upon the successful 

 destruction of the invading bacteria by the body cells, 

 especially the leukocytes. These devouring cells Metsch- 

 nikoff called phagocytes, and of them he recognized two 

 classes, the microphages, which are white blood-corpuscles, 

 and the macrophages, which are larger cells derived from 

 the endothelial and other tissues. Metschnikoff, his asso- 

 ciates, 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 1' Inflammation" 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 

 feed upon smaller organisms, some almost exclusively upon 

 bacteria, which they are able to kill and digest through an 

 intracellular enzyme demonstrated by Mouton,* and called 

 amebadiastase, and regarded as a form of trypsin. The 

 intracellular digestion of ccelenterate animals is accomplished 

 by means of actinodiastase, an enzyme discovered by 



*"Compte rendu de 1'Acad des Sciences de Paris," 1901, cxxxm, 

 p. 244. 



