IMMUNOLOGY 835 



to whether they are of myeloid or lymphoid origin. The lymphoid 

 cells of the blood and the cells of the lymph consist of various-sized 

 lymphocytes, which together with monocytes are termed agranulocytes. 

 The myeloid cells of the blood are the various granulocytes (heterophils 

 or polymorphonuclears, eosinophils, and basophils), the erythrocytes, 

 and the platelets. Some authors classify monocytes as lymphoid and 

 others as myeloid cells, but they are classified in this chapter as both, 

 since we believe that they arise from lymphocytes of the lymphatic tissue 

 and from hemocytoblasts (equivalent to lymphocytes) of the bone 

 marrow. 



The heterophils are functional in immunity by virtue of their obvious 

 phagocytic activities and probably because of their secretion of enzymes. 

 They are end cells, however, which do not reproduce or develop into 

 other cells. Lymphocytes and presumably monocytes, on the other hand, 

 can divide mitotically (PI. 4, Fig. 3) and both lymphocytes and mono- 

 cytes can develop into macrophages, with all of their developmental 

 potencies. Lymphocytes possess basophil cytoplasm and a relatively large, 

 deeply staining, often indented nucleus, with large acidophil nucleoli 

 PI. 3, Fig. 1. The monocytes may be the same size as, but in most cases 

 are larger than the medium lymphocytes, their cystoplasm is less basophil 

 and is increased in amount, and their nucleus is more vesicular, more 

 deeply indented with smaller chromatin granules and smaller and more 

 numerous nucleoli (cf. monocytoid lymphocyte in PI. 3, Fig. 1). As the 

 lymphocytes and monocytes transform into macrophages, they show in- 

 creased amounts of cytoplasm, their nuclei gradually take on macrophage 

 characteristics, and they become phagocytic (polyblasts 1-5, in PI. 3, 

 Fig. 2). These intermediate forms, together with lymphocytes, mono- 

 cytes, and macrophages, are grouped under the term lymphoid-macro- 

 phage system. 



There is general agreement that free lymphoid cells, more or less 

 similar to lymphocytes, occur in varying numbers under various physio- 

 logical and pathological conditions in the reticular tissues and the loose 

 connective tissue, and that in such sites they act as "stem" cells of 

 lymphoid and myeloid cells. The nature, classification, and even exact 

 morphology of these different stem cells are subject to such controversy 

 that they are termed by various authors lymphocytes, hemocytoblasts. 



