Figure 1318

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 efferent lymphatic vessel medulla

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 H8-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. xl8.

• Follicular dendritic cells with multiple, thin, hair-like branching cytoplasmic processes that interdigitate between B lymphocytes in the germinal centers (Fig. 13.21). 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 or months. 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. Follicular dendritic cells are not APCs, because they lack MHC II molecules.

The parenchyma of the lymph node is divided into a cortex and medulla (Fig. 13.22). The cortex forms the outer portion of the node except at the hilum. It consists of a dense mass of lymphatic tissue (reticular framework, fol licular dendritic cells, lymphocytes, macrophages, and plasma cells) and lymphatic sinuses, the lymph channels. The medulla is the inner part of the lymph node.

Lymphocytes in the superficial cortex are organized into nodules

As elsewhere, the lymphatic nodules of the cortex are designated primary nodules if they consist chiefly of small lymphocytes and secondary nodules if they possess a germinal center. Lymphatic nodules are found in the outer part of the cortex, called the superficial or nodular cortex. The portion of the cortex between the medulla and superficial cortex is free of nodules; it is called the deep cortex or paracortex. This region contains most of the T cells in the lymph node. Because of its dependence

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