In addition to lymph, lymphocytes also circulate through the lymph nodes. Although some lymphocytes enter nodes through afferent lymphatic vessels as components of lymph, most (about 90%) enter the node through the walls of postcapillary venules located in the deep cortex (see Fig. 14.21 and Plate 38, page 481). Because the postcapillary venules are lined by cuboidal or columnar endothelial cells, they are referred to as high endothelial venules (HEVs); Fig. 14.23). Cells of HEVs play an important role in circulating and concentrating lymph by transporting approximately 35% of fluid and electrolytes entering via afferent lymph vessels directly into the bloodstream. The cells of HEVs express a high concentration of water channels (aquaporin-1 [AQP-1] molecules). The rapid resorption of the interstitial fluid via water channels into the bloodstream causes lymph entering through the afferent lymph vessels to be drawn into the deep cortex by solvent drag.
These specialized endothelial cells also possess receptors for antigen-primed lymphocytes. They signal lymphocytes to leave the circulation and migrate into the lymph node. Both B and T cells leave the bloodstream through HEVs, crossing the endothelium by diapedesis—that is, by migrating between the endothelial cells in a manner similar to that described for neutrophils (see Fig. 10.6, page 277). The T cells remain in the thymus-dependent deep cortex; the B cells migrate to the nodular cortex (see Fig. 14.22). Most lymphocytes leave the lymph node by entering lymphatic sinuses from which they flow to an efferent lymphatic vessel.
The lymph node is an important site for phagocytosis and initiation of immune responses.
Phagocytosis of particulate material by phagocytotic cells within the lymph nodes is an important step in initiating an immune response. The physical accumulation of microorganisms and particulate substances conveyed in the lymph and phagocytosis of the particulate material help to concentrate antigen, thus enhancing its presentation to lymphocytes. Antigens conveyed in the lymph percolate through the sinuses and penetrate the lymph nodules to initiate an immune response. Some antigens become trapped on the surface of the follicular dendritic cells, whereas others are processed by macrophages, dendritic cells, and B cells, leading to activation and differentiation of B cells into antibody-producing plasma cells and memory B cells.
The plasma cells then migrate to the medullary cords where they synthesize and release specific antibodies into the lymph flowing through the sinuses. Plasma cells account for 1% to 3% of the cells in resting lymph nodules. Their number increases dramatically during an immune response, thereby increasing the amount of circulating immunoglobulins. Memory B cells may leave the lymph nodes and circulate to various regions throughout the body, where they can proliferate in response to subsequent exposure to their specific antigen. The presence of memory cells in various sites
FIGURE 14.23 • Photomicrograph of the deep cortex of a lymph node. This photomicrograph shows several longitudinally sectioned high endothelial venules (HEVs) as well as several that are seen in cross section (arrows). These vessels are lined by cuboidal endothelial cells. In some preparations, the walls of an HEV may be infiltrated with migrating lymphocytes, making it difficult to recognize. 400. Inset. The cross section of an HEV shown here at higher magnification reveals several lymphocytes (arrowheads) in the process of migrating from the HEV into the parenchyma of the lymph node. 640.
throughout the body ensures a more rapid response to an antigen, the secondary response.