lymph node

any of the glandlike masses of tissue in the lymphatic vessels containing cells that become lymphocytes. Also called lymph gland.

[1890-95]

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Small, rounded mass of lymphoid tissue contained in connective tissue.

They occur all along lymphatic vessels, with clusters in certain areas (e.g., neck, groin, armpits). They filter bacteria and other foreign materials out of lymph and expose them to lymphocytes and macrophages that can engulf them; these cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infections. The nodes also produce lymphocytes and antibodies, to be carried by lymph throughout the lymphatic system. In Hodgkin disease and other lymphomas, malignant lymph cells proliferate, causing lymph node enlargement. Other cancers often invade lymphatic vessels, which can carry cells from the tumour to lymph nodes, where they are trapped and grow into secondary tumours. Lymph nodes are therefore removed in cancer surgery to detect or prevent tumour spread.

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▪ anatomy

 any of the small, bean-shaped masses of lymphoid tissue enclosed by a capsule of connective tissue that occur in association with the lymphatic vessels. As part of the lymphatic system, lymph nodes serve as filters for the blood, providing specialized tissues where foreign antigens (antigen) can be trapped and exposed to cells of the immune system for destruction. They are typically found concentrated near junctions of the major lymphatic vessels, most prominently in the neck, groin, and armpits.

 Each lymph node is divided into two general regions, the capsule and the cortex. The capsule is an outer layer of connective tissue. Underlying the capsule is the cortex, a region containing mostly inactivated B and T lymphocytes (lymphocyte) plus numerous accessory cells such as dendritic cells and macrophages. The cortex is further divided into two functional areas: the outer cortex and inner cortex, or paracortex. These regions surround an inner medulla, which consists primarily of activated antibody-secreting plasma cells.

      Cells enter the lymph node through two primary routes. lymph and its associated cells enter through the afferent lymphatic vessels, which drain into each node through its convex surface. These vessels may drain directly from the lymphatic capillaries, or they may be connected to a previous node. Lymphocytes (lymphocyte) generally enter through specialized blood vessels called high endothelial venules (HEVs). HEVs contain a single layer of large endothelial cells that possess surface receptors specific for B and T lymphocytes. As these cells pass through the HEVs, they bind to the receptors and are carried into the paracortex of the lymph node.

      The structural divisions within a lymph node serve different purposes. Most of the lymphocytes within a node are “naive”—i.e., they have yet to encounter antigen—and therefore must migrate to regions where they will be most effective in recognizing foreign agents. B cells (B cell) enter the paracortex through the HEVs and then migrate into the outer cortex and join specialized dendritic cells and macrophages to form follicles. Primary follicles consist of a resting B cell surrounded by a loose network of dendritic cells. After encountering a foreign antigen, the B cell becomes activated and is surrounded by a more tightly packed association of dendritic cells and macrophages, forming a germinal centre. The germinal centre in turn is enclosed by a mantle zone—a ring of resting B cells and dendritic cells. The germinal centre and mantle together compose a secondary follicle, which is the site of antigen-dependent B-cell maturation. The activated B cells then migrate through the paracortex to the medulla, where they proliferate as antibody-secreting plasma cells. T cells enter the lymph node through the HEVs and remain in the paracortex, where the cortical macrophages and dendritic cells present antigenic peptides to the naive T cells, stimulating them to become activated helper T cells or cytotoxic T lymphocytes. All activated lymphocytes migrate through the medulla and enter the lymphatic circulation through the efferent lymphatic vessel, which drains either into adjacent lymph nodes or ultimately into the thoracic duct, a major vessel of the lymphatic system.

      The central role played by lymph nodes in filtering microorganisms and other undesired substances from the blood is critical to the functioning of the immune system but also makes lymph nodes vulnerable to cancer. As cancerous cells spread by metastasis, they can become trapped and concentrated in lymph nodes, where they proliferate. Virtually all cancers have the potential of spreading to lymph nodes, a condition that greatly complicates treatment. In most cases surgery alone will not remove the cancer from the nodes, and therefore postoperative radiation or chemotherapy is required.

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