io6 



Immunity 



Karl Roser* observed that the leukocytes of the bodies of higher 

 animals sometimes enclosed bacteria in their cytoplasm. Koch, 

 Sternberg, and others, confirmed the observation, but no attention 

 was paid to it until Metschnikofff correlated it with other known 

 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 Metschnikoff 

 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. 



Fig. 17. Phagocytosis; the omentum immediately after injection of typhoid 

 bacilli into a rabbit. Meshwork showing a macrophage, intermediate forms 

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



Metschnikoff, his associates, and his pupils soon collected evidence 

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

 fending 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 "Im- 

 munite 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; 



*"Beitrage zur Biologic niederster Organismen," Inaugural Dissertation, 

 Marburg, 1881. 



f'Virchbw's Archives," Bd. xcvi, p. 177; "Ann. de Plnst. Pasteur," 1887, 

 t. i, p. 321. 



