186 PHAGOCYTOSIS 



THE REVISED THEORY OF PHAGOCYTOSIS 



As previously stated, Metchnikoff has revised his theory from time 

 to time, as these discoveries were made on the influence of substances in 

 the body-fluids, not only upon phagocytosis itself, but also upon the 

 processes of immunity in general. He would regard extracellular cy- 

 tolysis (bacteriolysis, hemolysis, etc.) as due to the same ferments that 

 bring about the destruction and solution of the ingested bacterium or 

 other cell within the phagocyte, and, further, these extracellular fer- 

 ments are derived from the cells that are classed as phagocytes. By this 

 method of reasoning he would preserve the importance of the phago- 

 cytic theory. 



In local infections phagocytosis is usually well marked, and no 

 doubt plays an important part in resistance to and recovery from these 

 conditions. In general infections, however, as in bacteremias due to 

 staphylococci, streptococci, and particularly the typhoid and allied 

 bacilli, it is rare indeed to find evidence of a leukocyte in the blood- 

 stream becoming phagocytic. Here extracellular substances or anti- 

 bodies are chiefly operative in affording protection or in overcoming in- 

 fection. Even in local infections, where phagocytosis along the more 

 simple lines originally described by Metchnikoff is well marked, the 

 resistance of the bacteria is first attenuated or modified by the opsonins 

 of the body-fluids before the phagocytic process becomes well marked. 



The question, then, of the relative importance of the cellular and 

 humoral theories of immunity resolves itself to a consideration of the 

 origin of the substances in the body-fluids so potent in both processes. 

 If they are derived solely from the cells known to act as phagocytes, then 

 the cellular theory of phagocytosis, in its broader meaning, is the one 

 explanation of the processes of immunity as they are now understood. 

 This, however, has never been proved, and it is entirely likely that these 

 substances are products of a general, rather than of a more restricted, 

 cellular activity, so that ultimately all immunologic processes are cellular 

 in origin. For this reason we prefer to speak of the phagocytic cell in its 

 relation to immunity when dealing with the relation and activity of mi- 

 crophages and macrophages in a limited sense in the process of phago- 

 cytosis. 



