 Follicular dendritic cells (FDCs) have multiple, thin, hairlike branching cytoplasmic processes that interdigitate between B lymphocytes in the germinal centers (Fig. 14.20). Antigen–antibody complexes adhere to the dendritic cytoplasmic processes by means of the antibody’s Fc receptors, and the cell can retain antigen on its surface for weeks, months, or years. Although this mechanism is similar to the adhesion of antigen–antibody complexes to macrophages, the antigen is not generally endocytosed, as it is by the macrophage. FDCs are thus not APCs because they lack MHC II molecules.
General Architecture of the Lymph Node
The parenchyma of the lymph node is divided into a cortex and medulla (Fig. 14.21). The cortex forms the outer portion of the node except at the hilum. It consists of a dense mass of lymphatic tissue (reticular framework, dendritic cells, follicular dendritic cells, lymphocytes, macrophages, and plasma cells) and lymphatic sinuses, the lymph channels. The medulla is the inner part of the lymph node.
FIGURE 14.17 • Structure of a lymph node. a. This diagram depicts the general features of a lymph node as seen in a section. The substance of the lymph node is divided into a cortex, including a deep cortex, and a medulla. The cortex, the outermost portion, contains spherical or oval aggregates of lymphocytes called lymphatic nodules. In an active lymph node, nodules contain a lighter center called the germinal center. The medulla, the innermost region of the lymph node, consists of lymphatic tissue that appears as irregular cords separated by lymphatic medullary sinuses. The dense population of lymphocytes between the superficial cortex and the medulla constitutes the deep cortex. It contains the high endothelial venules. Surrounding the lymph node is a capsule of dense connective tissue from which trabeculae extend into the substance of the node. Under the capsule and adjacent to the trabeculae are, respectively, the subcapsular sinus and the trabecular lymphatic sinuses. Afferent lymphatic vessels (arrows) penetrate the capsule and empty into the subcapsular sinus. The subcapsular sinus and trabecular sinuses communicate with the medullary sinuses. The upper portion of the lymph node shows an artery and a vein and the location of the high endothelial venules of the lymph node. b. Photomicrograph of a lymph node in a routine H&E preparation. The dense outer portion of the lymph node is the cortex. It consists of aggregations of lymphocytes organized as nodules and a nodule-free deep cortex. The innermost portion of the lymph node, the medulla, extends to the surface at the hilum, where blood vessels enter or leave and where efferent lymphatic vessels leave the node. Surrounding the lymph node is the capsule, and immediately beneath it is the subcapsular sinus. 18.
FIGURE 14.18 • Photomicrograph of a lymph node. This silver preparation shows the connective tissue capsule (at the top), subcapsular sinus, and the superficial cortex of the lymph node (at the bottom). The reticular fibers (arrows) form an irregular anastomosing network throughout the stroma of the lymph node. Note elongated oval nuclei of reticular cells (arrowheads), which are in intimate contact with reticular fibers in the sinus. 640.