IMMUNITY 129 



antigen being accomplished by the ferment-like substance, the 

 complement. 



Ehrlich's conception of immunity, then, was that of a chemist, 

 and his antibodies the result of a chemical union between the body 

 cells and the foreign protein, bacterial or other. 



" The Cellular Theory." Metchnikoff as a biologist naturally 

 based his theory on his observations of biological phenomena. 

 Since all cells that have amebic motion are capable of enveloping 

 small particles he considered that certain body cells, and partic- 

 ularly the leukocytes, might rid the body of infective material 

 and thus bring about a state of immunity. He likened these 

 cells to scavengers and gave to them the name of phagocytes, 

 because, according to his theory, they are during an infection 

 mainly occupied in picking up and disposing of offensive material. 



He divided the phagocytes into two classes; in one he placed 

 the polynuclear leukocytes and gave them the name of micro- 

 phages, and in the other he placed the endothelial cells, the mono- 

 nuclear leukocytes, and embryonic connective tissue cells. To these 

 he gave the name macrophages. The former play an active role 

 in acute pyogenic infections, the latter are most active in chronic 

 bacterial infections such as tuberculosis and syphilis. Metchnikoff 

 soon realized that phagocytosis alone could not explain all the 

 phenomena and that further study was necessary. He found 

 that the digestive power of the phagocytes is very great, and 

 that gradually they are able to dissolve almost any substance. 

 He considered two of these substances, one derived from the 

 microphages which he called microcytose, and the other obtained 

 from the macrophages and to which he gave the name of macro- 

 cytose, to be identical with Ehrlich's interbody and complement. 



The relative importance of the cellular and humoral theories 

 rests largely on which of the body cells are most active in forming 

 antibodies. Metchnikoff showed the important part played 

 by phagocytes in any infection, and claimed that the antibodies 

 in the circulating fluids are the products of their activity. Ehrlich, 

 on the other hand, emphasized the presence of immune bodies 

 in the body fluids and sought to explain the method by which they 



