PHAGOCYTOSIS 247 



fresh cells, in either case leading to the production of free and 

 motile endothelial cells, which have the closest resemblance to 

 the large hyaline cells of the blood, though they may be much 

 larger. These cells are most active and important phagocytes, 

 especially in the peritoneum. They have also the power of 

 undergoing organization, especially into fibrous tissue, and many, 

 if not all, of the fibroblasts of granulation tissue are endothelial in 

 origin. Further, some at least of the giant cells so familiar in 

 chronic inflammatory processes are derived from the endothelial 

 cells of the lymph clefts and lymph capillaries. This has been 

 proved to demonstration by Bergengriin in the case of the giant 

 cells in leprosy. Our knowledge of the origin of the giant cells of 

 tubercle is less exact, but analogy with those of leprosy would 

 lead us to infer that they are endothelial also. In both diseases 

 the giant cells are most important phagocytes. 



Epithelial cells only exceptionally act as phagocytes. We 

 have already referred to the ingestion of fat by the columnar cells 

 of the intestine, and the other important example is supplied by 

 the epithelial cells lining the alveoli of the lungs. These are 

 flattened plaques under normal conditions, but in the presence of 

 an irritant they become cuboidal or columnar, detach themselves, 

 or bud off similar cells, and are powerful phagocytes. These are 

 the dust cells so frequently seen in sputum. 



The nature of the cells which take part in phagocytosis is 

 determined to some extent by the nature of the irritant. Thus, 

 when the pyogenic bacteria are ingested it is usually by poly- 

 nuclear leucocytes, whereas it is extremely rare to find these 

 cells containing tubercle bacilli in the tissues, though they will 

 take them up readily enough under the artificial conditions of 

 opsonic experiments. Metchnikoff classifies phagocytes into two 

 groups macrophages and microphages. His description of these 

 cells is not absolutely clear, but in general the microphages 

 correspond to the polynuclear leucocytes, and the macrophages to 

 the large hyaline cells of the blood and the endothelial cells of 

 the serous sacs and connective tissues. He claims that the 

 former are especially concerned with the phagocytosis of bacteria, 

 the latter with red blood-corpuscles and similar objects. This 

 distinction is not a valid one, since endothelial cells are ex- 

 tremely active phagocytes for bacteria, and polynuclear cells will 

 ingest red corpuscles with great readiness when provided with 

 a suitable opsonin (see Fig. 49). It would appear that under 



