CHAPTER XV. 

 PHAGOCYTOSIS. OPSONINS AND BACTERIOTROPINS. 



i. Phagocytosis. 



By phagocytosis is meant the taking up, or engulfing of foreign substances 

 by certain cells (digestive cells or phagocytes) for the purposes of digestion. 

 As a mode of nutrition, this is well known to exist, normally, in the lowest 

 unicellular animals as for instance the amebae. Intracellular digestion can, 

 however, be traced to organisms higher in the scale of the animal kingdom, 

 and even among mammals the function of cell ingestion is found, although 

 limited in a sense, to a definite group of cells, especially those derived from 

 the mesoderm. 



The inspiration for the work on phagocytosis and the greater part of its 

 theoretical considerations have emanated from Metschnikoff and his numer- 

 ous pupils at the Pasteur Institute at Paris. Metschnikoff divides the 

 phagocytes into two classes, the "sessile or fixed phagocytes," and the 

 "wandering phagocytes." The first is the stationary endothelial lining of 

 blood vessels, and lymph spaces, as well as the large cells of the spleen pulp 

 and lymph glands; the second, consists of the white blood cells of the circula- 

 tion. From another standpoint the phagocytes are divided into "micro- 

 phages" and "macrophages." The former are practically identical with 

 the neutro- and eosinophile polymorphonuclear leucocytes, while the latter 

 present no distinct group, but include large lymphocytes, myelocytes, giant 

 cells, etc. The cells designated as sessile phagocytes also belong to the 

 class of macrophages. The size of the cell was considered by Metschnikoff 

 as the deciding feature; not all macrophages are mononuclear as generally 

 believed. Thus for example macrophages appearing in the peritoneal 

 fluid of guinea-pigs frequently possess, like the giant cells of the tubercle, 

 numerous nuclei. According to Metschnikoff it is primarily the micro- 

 phages to whom the function of bacterial phagocytosis is allotted, while the 

 macrophages serve for the purpose of ingesting dead or moribund tissue 

 structure. Still there are certain pathogenic micro-organisms, tubercle 

 bacilli, lepra bacilli, actinomyces, which are favored in being digested by the 

 selective macrophages. The evidence of phagocytosis is established by 

 mixing either in vitro or vivo the substance for phagocytosis, plus the phago- 

 cytes, and noting the changes which ensue; [either in a stained or unstained 

 preparation]. The phagocytes of the guinea-pig's peritoneal cavity are 



