Lymph node

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Image:Illu lymph node structure.png Lymph nodes, also incorrectly called "Lymph glands" (they do not secrete anything and are therefore not glands), are components of the lymphatic system. Lymph nodes act as filters, with an internal honeycomb of connective tissue filled with lymphocytes that collect and destroy bacteria and viruses. When the body is fighting an infection, these lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes. Humans have approximately 500-600 lymph nodes. Clusters of lymph nodes are found in the underarms, groin, neck, chest, and abdomen.

The spleen and tonsils are large lymphoid organs that serve similar functions, though the spleen filters out blood cells rather than bacteria or viruses.

Lymph nodes are bean-shaped and range in size from a few millimeters to about 1-2 cm in their normal state. They may become enlarged due to a tumor or infection. In some cases, they may feel enlarged due to past infections; although one may be perfectly healthy, he may still feel residually enlarged nodes. The lymph node is surrounded by a fibrous capsule, and inside the lymph node the fibrous capsule extends to form trabeculae. Thin reticular fibers form a supporting meshwork inside the node. The concave side of the lymph node is called the hilum. The artery and vein attach at the hilum and allow blood to enter and leave the organ, respectively. The parenchyma of the lymph node is divided into an outer cortex and an inner medulla. The cortex is absent at the hilum.

The cortex contains several oval-shaped lymphoid nodules (also called follicles) which are aggregates of lymphocytes contained within a meshwork of supporting cells. Nodules that consist mainly of small lymphocytes are called primary nodules. Nodules called secondary nodules are those that contain a pale central region called a germinal center. The germinal center forms (and thus a primary nodule becomes a secondary nodule) when a B cell that has recognized an antigen undergoes proliferation, differentiates into plasma cells, and forms antibodies.

The band of the cortex adjacent to the medulla is the deep cortex, also called the juxtamedullary cortex or paracortex. This layer is devoid of nodules. Formation of the deep cortex depends on the migration of T cells. Thus it is sometimes also called the thymus-dependent cortex. In comparison, the layer of the cortex that contains nodules is called the nodular cortex.

The medulla consists of cords of lymphatic tissue (medullary cords) separated by vessel-like spaces called medullary sinuses.

Lymph travels to the lymph node via afferent lymphatic vessels and drains into the node just beneath the capsule in a space called the subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the pseudopods of macrophages which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the efferent lymphatic vessel.

Lymphocytes, both B cells and T cells, constantly circulate through the lymph nodes. They enter the lymph node via the bloodstream and may cross the wall of the bloodvessel by the process of diapedesis. The B cells migrate to the nodular cortex and medulla, and the T cells migrate to the deep cortex.

When a lymphocyte recognizes an antigen, B cells become activated and migrate to germinal centers. When antibody-producing plasma cells are formed, they migrate to the medullary cords. Stimulation of the lymphocytes by antigens can accelerate the migration process to about 10 times normal, resulting in characteristic swelling of the lymph nodes.

Image:Lymphatic system.gif

1 Lymph nodes of the head and neck
2 See also
3 External links

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Lymph nodes of the head and neck

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Anterior cervical

These nodes, both superficial and deep, lie above and beneath the sternocleidomastoid muscles. They drain the internal structures of the throat as well as part of the posterior pharynx, tonsils, and thyroid gland.

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Posterior cervical

These nodes extend in a line posterior to the sternocleidomastoids but in front of the trapezius, from the level of the mastoid bone to the clavicle. They are frequently enlarged during upper respiratory infections.

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Tonsillar

These nodes are located just below the angle of the mandible. They drain the tonsilar and posterior pharyngeal regions.

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Sub-mandibular

These nodes run along the underside of the jaw on either side. They drain the structures in the floor of the mouth.

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Sub-mental

These nodes are just below the chin. They drain the teeth and intra-oral cavity.

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Supraclavicular

These nodes are in the hollow above the clavicle, just lateral to where it joins the sternum. They drain a part of the thoracic cavity and abdomen. Virchow's node is a left supraclavicular lymph node which receives the lymph drainage from most of the body (especially the abdomen) via the thoracic duct and is thus an early site of metastasis for various malignancies.

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