RELATION OF THE BODY-FLUIDS TO PHAGOCYTOSIS 185 



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 r61es 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 has shown, are undergoing more 

 specific changes. Thus, for instance, the leukocytes from a pneumonia 

 patient were found more vigorous against invasion of the pneumococcus 

 than are those from a normal person, regardless of the influence of serum. 



When a microparasite is ingested, the process has only begun. Un- 

 less suitable endolysins are present and the endotoxin is absorbed or 

 otherwise dealt with, and unless suitable digestive enzymes are secreted 

 and the bacterium is dissolved, the process is useless, or, indeed, if viable 

 bacteria are transported to other parts of the body, it may be dangerous. 



