188 PHAGOCYTOSIS 



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. 



THE REVISED THEORY AND ROLE OF PHAGOCYTOSIS IN IMMUNITY 



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 con- 

 ditions. Recent investigations by Bull 1 have shown that following 

 agglutination of various bacteria in vivo, phagocytosis of the micro- 

 organisms frequently follows. In infections due to the various patho- 

 genic micrococci, as staphylococci, pneumococci, and streptococci, 

 phagocytosis appears to be most active and an important means of 

 overcoming the infection. In other infections, as those due to the ty- 

 phoid bacillus and allied bacilli, it is probable that extracellular sub- 

 stances or antibodies are chiefly operative in affording protection or in 

 overcoming infection, although certain of these, as the agglutinins and 

 opsonins, facilitate phagocytosis. 



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. 

 1 Jour. Infect. Dis., 1915, 16, 109. 



