Lymph node

A lymph node or lymph gland is an ovoid or kidney-shaped organ of the lymphatic system and the adaptive immune system. Lymph nodes are widely present throughout the body and are linked by the lymphatic vessels as a part of the circulatory system. They are major sites of B and T lymphocytes and other white blood cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells, but they do not have a detoxification function.Lymphatic systemSection of small lymph node of rabbit.Lymphatics of the armLymphatics of the axillary regionLymph node histology A lymph node or lymph gland is an ovoid or kidney-shaped organ of the lymphatic system and the adaptive immune system. Lymph nodes are widely present throughout the body and are linked by the lymphatic vessels as a part of the circulatory system. They are major sites of B and T lymphocytes and other white blood cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells, but they do not have a detoxification function. In the lymphatic system a lymph node is a secondary lymphoid organ. A lymph node is enclosed in a fibrous capsule and is made up of an outer cortex and an inner medulla. Lymph nodes become inflamed or enlarged in various diseases, which may range from trivial throat infections to life-threatening cancers. The condition of lymph nodes is very important in cancer staging, which decides the treatment to be used and determines the prognosis. When inflamed or enlarged, lymph nodes can be firm or tender. Lymph nodes are kidney or oval shaped and range in size from a few millimeters to about 1–2 cm long. Each lymph node is surrounded by a fibrous capsule, which extends inside a lymph node to form trabeculae. The substance of a lymph node is divided into the outer cortex and the inner medulla. The cortex is continuous around the medulla except where the medulla comes into direct contact with the hilum. Thin reticular fibers of reticular connective tissue and elastin form a supporting meshwork called reticulin inside the node. B cells are mainly found in the outer (superficial) cortex where they are clustered together as follicular B cells in lymphoid follicles, and T cells are mainly found in the paracortex. A lymph node is divided into compartments called lymph nodules (or lobules), each consisting of a cortical region of combined follicle B cells, a paracortical region of T cells, and a basal part of the nodule in the medulla. The number and composition of follicles can change when challenged by an antigen, which causes follicles to develop a germinal center. Elsewhere in the node, there are only occasional leukocytes. As part of the reticular network, there are follicular dendritic cells in the B cell follicle and fibroblastic reticular cells in the T cell cortex. The reticular network provides structural support and a surface for adhesion of the dendritic cells, macrophages and lymphocytes. It also allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells. Lymph enters the convex side of a lymph node through multiple afferent lymphatic vessels and flows through spaces called sinuses. A lymph sinus, which includes the subcapsular sinus, is a channel within the node lined by endothelial cells along with fibroblastic reticular cells, allowing for the smooth flow of lymph. The endothelium of the subcapsular sinus is continuous with that of the afferent lymph vessel and also with that of the similar sinuses flanking the trabeculae and within the cortex. All of these sinuses drain the filtered lymphatic fluid into the medullary sinuses, from which the lymph flows into the efferent lymph vessels to exit the node at the hilum on the concave side. These vessels are smaller and don't allow the passage of macrophages so that they remain contained to function within a lymph node. In the course of the lymph, lymphocytes may be activated as part of the adaptive immune response. The lymph node capsule is composed of dense irregular connective tissue with some plain collagenous fibers, and a number of membranous processes or trabeculae extend from its internal surface. The trabeculae pass inward, radiating toward the center of the node, for about one-third or one-fourth of the space between the circumference and the center of the node. In some animals they are sufficiently well-marked to divide the peripheral or cortical portion of the node into a number of compartments (nodules), but in humans this arrangement is not obvious. The larger trabeculae springing from the capsule break up into finer bands, and these interlace to form a mesh-work in the central or medullary portion of the node. In these trabecular spaces formed by the interlacing trabeculae contain the proper lymph node substance or lymphoid tissue. The node pulp does not, however, completely fill the spaces, but leaves between its outer margin and the enclosing trabeculae a channel or space of uniform width throughout. This is termed the subcapsular sinus (lymph path or lymph sinus). Running across it are a number of finer trabeculae of reticular connective tissue, the fibers of which are, for the most part, covered by ramifying cells. The subcapsular sinus (lymph path, lymph sinus, marginal sinus) is the space between the capsule and the cortex which allows the free movement of lymphatic fluid and so contains few lymphocytes. It is continuous with the similar lymph sinuses that flank the trabeculae.