VARIETIES OF PHAGOCYTES 179 



red corpuscles, cellular debris, inorganic particles, such as coal-dust, and 

 even soluble substances, such as bacterial toxins. 



Subsequent discoveries have shown that many other factors are 

 present that considerably modify the workings of so simple a process. 

 Metchnikoff has, therefore, modified his theory from time to time as 

 new discoveries were made, but has always preserved the primary im- 

 portance of the phagocyte itself. 



Before stating the revised theory as it stands today, we will describe 

 the kind of cells that may act as phagocytes, and consider the methods 

 and reasons why these cells assume the functions of phagocytes. 



VARIETIES OF PHAGOCYTES 



Not only leukocytes, but other body-cells, have been found active 

 in the processes of phagocytosis. Metchnikoff has divided the phago- 

 cytes into two great classes : 



1. Microphages principally the polynuclear neutrophilic leukocytes. 

 (See Fig. 46.) The eosinophilic leukocytes are also included in this group, 

 but are of doubtful importance and weak in phagocytic powers. The 

 small lymphocyte may be included in this class, although it is usually con- 

 sidered with the second group. 



2. Macrophages, principally the large mononuclear leukocytes; 

 ameboid cells of the spleen and lymphatic glands; alveolar cells of the 

 lung; endothelial cells of the serous cavities and lymph-spaces; bone- 

 corpuscles and giant-cells of bone-marrow and embryonic connective- 

 tissue cells (Figs. 44 and 45). As shown experimentally by Rous and 

 Jones the phagocytic powers of fibroblasts appear to be very feeble. 1 



The most important are the leukocytes, especially the polynuclear 

 leukocytes and the large lymphocytes of the blood. All the leukocytes, 

 however, have phagocytic powers, as is well seen in opsonic determina- 

 tions. Eosinophiles are seldom known to ingest bacteria, but in in- 

 fections with animal parasites, or after the injection of extracts of ani- 

 mal parasites, both a local and a general increase in eosinophilous forms 

 may be observed. 



Small lymphocytes are much less active than the large, presumably 

 because they contain less of the mobile cytoplasm, and consist chiefly 

 of the structurally fixed nuclear substance, and while they take up but 

 a small number of bacteria, they may be observed to contain various 

 other cells, such as red corpuscles and cellular de*bris. 



Besides the leukocytes, some of the tissue-cells which are free 01 

 1 Jour. Exper. Med., 1917, 25, 189. 



