112 Immunity 



facts and original observations, and came to the conclusion that the 

 enclosed -bacteria had been eaten by the leukocytes in which they 

 were killed 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. The original 

 conception upon which this "theory of phagocytosis" was founded, 

 . refers recovery in many, if not all of the infectious diseases, to the 

 successful destruction of the invading bacteria by the body cells, 

 especially the leukocytes. These devouring cells Metchnikoff 

 called phagocytes, and of them he recognized two classes, the micro- 

 phages, which are white blood-corpuscles, and the macrophages, 

 which are larger cells derived from the endothelial and other tissues. 

 Metchnikoff, his associates, and his pupils soon collected evidence 

 sufficient to show that phagocytosis, if not the chief factor in defending 



Fig. 19. — Phagocytosis; the omentum immediately after injection of typhoid 

 bacilli into a rabbit. Meshwork showing a macrophage, intermediate form and 

 a trailer, all containing intact bacilli (Buxton and Torry). 



the body from infectious organisms, is at least an important 

 one. Many of the most interesting facts are described in Metchni- 

 koff 's books, "Etudes sur ITnflammation " 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 leukocytes are active in 

 their phagocytic behavior toward them; that in acquired immunity, 

 the leukocytes previously 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 



