836 IMMUNOLOGY 



lymphoblasts, myeloblasts, monoblasts, and so forth, according to the 

 particular theory of blood formation held by the author (see Bloom, 

 1938). We have adopted essentially the unitarian viewpoint of Maxi- 

 mow and have called all free mesenchymal stem cells, with wide poten- 

 cies for development, hemocytoblasts in the bone marrow and lympho- 

 cytes in all other locations. Lymphocytes and hemocytoblasts are identical 

 morphologically and probably in their developmental potencies. Under 

 physiological conditions, lymphocytes in lymphatic tissue give rise only 

 to lymphocytes (PI. 4), and hemocytoblasts in bone marrow give rise 

 only to myeloid cells (erythroblasts, myelocytes, and so forth), but un- 

 der abnormal stimuli they may exhibit their full potencies for develop- 

 ment. In general, these free stem cells are self-perpetuating, but they 

 may arise from the fixed mesenchymal cells of the preceding section. 



C. So-called Systems of Cells. — The foregoing classification of cells 

 should be brought into line with the so-called systems of cells frequently 

 used by various authors. Modern concepts of the cellular basis of im- 

 munity have been largely based on studies of inflammation. Credit should 

 be given to Metschnikoff (1892) for insisting upon the essential role of 

 the mesenchymal cells in inflammation and to Cohnheim, Ziegler, Mar- 

 chand (1924, review), and Maximow (1927a, 1927b, review), among 

 others, for studying the histogenesis of the local inflammatory reactions. 

 Metschnikoff (1892 and 1905, among other studies) laid the whole 

 foundation for the modern concept of the defense function of fixed 

 and mobile cells of the connective tissue by phagocytosis. His concept 

 was essentially physiological. He distinguished ( 1 ) microphages, herein 

 designated heterophils; and (2), macrophages, which are identical with 

 macrophages as herein defined, except that he included the phagocytic 

 microglial cells of the brain, which are possibly of mesenchymal origin. 

 The modern understanding of macrophages is based largely upon the 

 studies of vital staining and the storage of colloidal dyes, chiefly by 

 Renaut, Maximow, Goldman, Tschaschin, Kiyono, and Aschoff. The 

 Gefdsswandzellen of the Marchand-Herzog school (see Marchand, 

 1924) include pericytes and perivascular macrophages (adventitial cells) 

 which are supposed to arise from the endothelium of developing vessels. 

 Aschoff 's (1924, review )retiailo-endothelial system, broadly defined, 

 consists of the macrophages as we have outlined them. 



It has unfortunately been assumed by most writers that the increase 



