178 PHAGOCYTOSIS 



inflammatory reaction impressed him as significant and similar to the 

 movements of the ameba in its process of feeding, and led to com- 

 parative studies in inflammation in the lower forms of life, where 

 processes were much simpler to watch and may indicate what occurs 

 in the higher vertebrates. 



He found that when daphnias are invaded by the spores of a yeast, 

 the monospora, they may multiply in the body of the host and bring 

 about its destruction. When, however, a few spores gained access, he 

 found that the daphnia's leukocytes approached them, formed a wall 

 around them, and finally brought about their destruction by a process 

 of digestion. He also observed that if rose prickles were stuck into 

 large bipinnaria larvae of star fish, these were soon enveloped in a mass 

 of ameboid cells. From this he concluded that, in inflammation, the 

 exudation of leukocytes may be regarded as a reaction against any sort 

 of injury, whether mechanical or due to bacterial invasion. 



Metchnikoff traced this defensive reaction against an invading 

 microorganism from small invertebrates up to man, and showed that 

 instead of bacteria attacking leukocytes and forcing a passage into them, 

 as was then believed, they were indeed pursued and engulfed by the 

 leukocytes. Connecting his various discoveries, he was able to formu- 

 late the idea that "the intracellular digestion of unicellular organisms 

 and of many invertebrates had been hereditarily transmitted to the 

 higher animals, and retained in them by the ameboid cells of mesodermic 

 origin. These cells, being capable of ingesting and digesting all kinds 

 of histologic elements, may apply the same power to the destruction of 

 microorganisms." To a cell, and especially to a leukocyte, possessing 

 this activity and power he applied the name phagocyte, because of its 

 ability to act as a scavenger, gathering up the living and dead material. 



THE ORIGINAL THEORY OF PHAGOCYTOSIS 



The theory as originally adduced by Metchnikoff regarded the leu- 

 kocytes and certain other cells as specific fighting cells, able to engulf 

 and consume living as well as dead bacteria and cellular debris. The 

 outcome of any infection would depend upon the success or failure of the 

 phagocytes to overcome the invaders: if they were successful in in- 

 gesting all the bacteria, their victory meant recovery; if, on the other 

 hand, they were destroyed by the invaders, the host was overcome by 

 the infection. 



Phagocytes may ingest not only living and dead bacteria, but also 



