RELATION OF THE BODY-FLUIDS TO PHAGOCYTOSIS 187 



in which phagocytosis was observed and known to be of great importance, 

 the bacteria are first prepared for phagocytosis by substances in the 

 body-fluids, and that without this preliminary preparation of the bac- 

 teria phagocytosis was slight and of little consequence. 



Metchnikoff corroborated most of these discoveries, and modified 

 his theory from time to time to meet the developments and keep them 

 within the limits of the phagocytic theory. For example, when bac- 

 teriolysis was shown by Bordet to be due to two separate substances in 

 the body-fluids, which he called substance sensibilisatrice and alexin 

 (later renamed by Ehrlich amboceptor and complement respectively), 

 Metchnikoff claimed that this phenomenon was extracellular digestion, 

 similar to the intracellular digestion that occurs within the phagocyte, 

 and brought about by ferments secreted and liberated from leukocytes 

 or other cells classed as phagocytes. He regards alexin as a cytase se- 

 creted by leukocytes, or liberated upon their disintegration; similarly 

 the substance sensibilisatrice is regarded as a free ferment (fixateur), 

 derived principally from leukocytes, and concerned in preparing the 

 bacterial or other cell for the digestive-like action of the cytase. 



Aside from these free ferments that are capable of producing extra- 

 cellular lysis, Metchnikoff has long known that other substances that 

 aid phagocytosis itself may be present in the body-fluids; he regards 

 these as of the nature of stimulins, or substances that stimulate leuko- 

 cytes to become more actively phagocytic. On the other hand, Leish- 

 man, Wright and Douglas, Neufeld, and Rimpau, Hektoen, and others 

 have clearly demonstrated that they facilitate phagocytosis, not by 

 stimulating the leukocytes, but rather by lowering the resistance of 

 bacteria or in some way rendering them more vulnerable to phagocytosis 

 (opsonins, bacteriotropins) . 



Thus the gap between the original cellular theory, which ascribed 

 protection and cure to phagocytosis pure and simple, and the humoral 

 theory (finally summed up by Ehrlich in his side-chain theory), which 

 ascribed the chief and primary roles to substances in the body-fluids, 

 and relegated phagocytes to a position of secondary importance, re- 

 garding them only as scavengers that remove dead or disabled micro- 

 organisms, has been filled with discoveries correlating both processes. 



The vitality of the leukocyte is to be regarded as important in the 

 consideration of phagocytosis as a means of defense. While the body- 

 fluids are acting upon the invaders, the leukocytes themselves are prob- 

 ably undergoing quantitative and qualitative changes. They are in- 

 creasing in numbers, and, as Rosenow 1 has shown, are undergoing more 

 1 Jour. Infect. Dis., 1906, 3, 683. 



