CHAPTER XV. 

 PHAGOCYTOSIS. OPSONINS AND BACTERIOTROPINS. 



I. Phagocytosis. 



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

 stances by certain cells (digesting 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, for instance the amebae. Intracellular 

 digestion, however, can be traced to organisms higher in the scale of the 

 animal kingdom; even among mammals the function of cell ingestion is 

 found, although limited 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 Metchnikoff and his 

 numerous pupils at the Pasteur Institute at Paris. Metchnikoff 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 

 circulation. From another standpoint the phagocytes are divided into 

 ' ' microphages ' ' 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 Metch- 

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

 generally believed. Thus macrophages appearing in the peritoneal fluid of 

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

 ous nuclei. According to Metchnikoff it is primarily the microphages 

 to whom the function of bacterial phagocytosis is allotted, while the macro- 

 phages serve for the purpose of ingesting dead or moribund tissue struc- 

 ture. Still there are certain pathogenic micro-organisms, tubercle bacilli, 

 lepra bacilli, actinomyces, which are favored in that they also are digested 

 by the selective macrophages. The evidence of phagocytosis is established 

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

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

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

 cavity are especially well adapted for the study of phagocytosis in vivo. 

 The following experiment of Metchnikoff may serve as a type. 



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