IMMUNOLOGY 833 



functional aspect of immunity, the predominantly fixed cells of the 

 reticular and loose connective tissues may be divided into two great 

 groups: (1) fixed and free macrophages (including the reticular cells), 

 and (2) the fibroblasts of connective tissue and the endothelial cells 

 lining the ordinary blood vessels. 



The term macrophage is essentially a physiological designation for 

 almost any large mononuclear connective-tissue cell which is or may 

 become phagocytic. Under macrophages are classified a group of fixed 

 mesenchymal cells, which retain many embryonic characters and a wide 

 range of potencies for development. The concept that the connective 

 tissue of the adult body possesses fixed cells retaining mesenchymal or 

 embryonic potencies for development is largely due to Marchand (1924, 

 review) and Maximow (1927a, review). There are three chief cate- 

 gories: (1) Pericytes (Maximow) which are fixed, undifferentiated, 

 outstretched cells in the adventitia of all of the small blood vessels of 

 loose connective tissue throughout the body; (2) reticular cells, which, 

 together with fibers, form the stroma of all reticular (myeloid and 

 lymphatic) tissues; (3) littoral cells (Siegmund), which line the sinuses 

 or sinusoids of the reticular tissues, the liver, hypophysis, and adrenal 

 (Pis. 1 and 2). Where phagocytic in the liver, they are generally desig- 

 nated Kupffer cells (PI. 1, Fig. 1; PI. 2, Fig. 1). The cells lining the 

 sinuses of the reticular tissues are actually reticular cells. The littoral 

 cells are often called endothelial cells or cells of the special endothelium, 

 but this is unfortunate because the littoral cells have wide developmental 

 potencies, whereas the ordinary endothelial cells lining the blood vessels 

 have restricted developmental potencies. 



There is general agreement that under proper stimuli the cells of 

 these three categories can divide by mitosis, can become phagocytic, 

 can develop into fibroblasts, or can develop into practically any other 

 type of cell of the blood or connective tissue. From the standpoint of the 

 present discussion, it is important that they can become phagocytic either 

 in their fixed position (fixed macrophages) or after rounding up and 

 becoming free (free macrophages). It is not definitely known, however, 

 whether, while engorged, they temporarily or permanently lose their 

 mesenchymal potencies. There may be a difference, for example, between 

 the primitive outstretched reticular cell and the same cell after it has 

 become free and phagocytic. 



