Lymph node

; 4) Lymphoid nodule; 5) Trabeculae|alt= Lymph nodes are kidney or oval shaped and range in size from 2 mm to 25 mm on their long axis, with an average of 15 mm. Each lymph node is surrounded by a fibrous capsule (made of collagenous connective tissue), which extends inside the lymph node to form trabeculae. The substance of a lymph node is divided into the outer cortex and the inner medulla. The hilum is an indent on the concave surface of the lymph node where lymphatic vessels leave and blood vessels enter and leave. There are no lymph nodes in the central nervous system, which is separated from the body by the blood–brain barrier. Lymph from the meningeal lymphatic vessels in the CNS drains to the deep cervical lymph nodes. However, the CNS does innervate lymph node by sympathetic nerves. These regulate lymphocyte proliferation and migration, antibody secretion, blood perfusion, and inflammatory cytokine production. Size Subdivisions , showing dark, light, mantle and marginal zones A lymph node is divided into compartments called nodules (or lobules), each consisting of a region of cortex with combined follicle B cells, a paracortex of T cells, and a part of the nodule in the medulla. The outer cortex consists of groups of mainly inactivated B cells called follicles. The medulla contains large blood vessels, sinuses and medullary cords that contain antibody-secreting plasma cells. There are fewer cells in the medulla. 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 flow Lymph enters the convex side of a lymph node through multiple afferent lymphatic vessels, which form a network of lymphatic vessels () and flows into a space () underneath the capsule called the subcapsular sinus. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells, the former of which, along with T and B cells, become activated in the presence of antigens through lymphatic flow. The fewer efferent vessels allow this flow to be slowed, providing time to activate and distribute a larger number of immune cells in the event of an infection. A lymph node contains lymphoid tissue, i.e., a meshwork or fibers called ' with white blood cells enmeshed in it. The regions where there are few cells within the meshwork are known as '. It is lined by reticular cells, fibroblasts and fixed macrophages. Capsule Thin reticular fibers (reticulin) of reticular connective tissue form a supporting meshwork inside the node. 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. 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 fibers, mostly covered by ramifying cells. Inverted lymph nodes Some mammal species, such as pigs, rhinoceroses, hippopotamuses, and certain cetaceans, have so-called "inverted" lymph nodes. In these nodes, the afferent lymph carries antigens from the center (where the B-cell follicles are located) toward the periphery. Mature B and T lymphocytes exit the lymph node from its periphery directly into the general bloodstream via efferent venules. These structural differences do not appear to impact the functionality of the lymph nodes. == Function ==

In the lymphatic system, a lymph node is a secondary lymphoid organ. s The primary function of lymph nodes is the filtering of lymph to identify and fight infection. In order to do this, lymph nodes contain lymphocytes, a type of white blood cell, which includes B cells and T cells. These circulate through the bloodstream and enter and reside in lymph nodes. B cells produce antibodies. Each antibody has a single predetermined target, an antigen, that it can bind to. These circulate throughout the bloodstream and if they find this target, the antibodies bind to it and stimulate an immune response. Each B cell produces different antibodies, and this process is driven in lymph nodes. B cells enter the bloodstream as "naive" cells produced in bone marrow. After entering a lymph node, they then enter a lymphoid follicle, where they multiply and divide, each producing a different antibody. If a cell is stimulated, it will go on to produce more antibodies (a plasma cell) or act as a memory cell to help the body fight future infection. If a cell is not stimulated, it will undergo apoptosis and die. Antigens are molecules found on bacterial cell walls, chemical substances secreted from bacteria, or sometimes even molecules present in body tissue itself. These are taken up by cells throughout the body called antigen-presenting cells, such as dendritic cells. These antigen presenting cells enter the lymph system and then lymph nodes. They present the antigen to T cells and, if there is a T cell with the appropriate T cell receptor, it will be activated. B cells acquire antigen directly from the afferent lymph. If a B cell binds its cognate antigen it will be activated. Some B cells will immediately develop into antibody secreting plasma cells, and secrete IgM. Other B cells will internalize the antigen and present it to follicular helper T cells on the B and T cell zone interface. If a cognate FTh cell is found it will upregulate CD40L and promote somatic hypermutation and isotype class switching of the B cell, increasing its antigen binding affinity and changing its effector function. Proliferation of cells within a lymph node will make the node expand. Lymph is present throughout the body, and circulates through lymphatic vessels. These drain into and from lymph nodesafferent vessels drain into nodes, and efferent vessels from nodes. When lymph fluid enters a node, it 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 towards either a more central lymph node or ultimately for drainage into a central venous subclavian blood vessel. • The B cells migrate to the nodular cortex and medulla. • The T cells migrate to the deep cortex. This is a region of a lymph node called the paracortex that immediately surrounds the medulla. Because both naive T cells and dendritic cells express CCR7, they are drawn into the paracortex by the same chemotactic factors, increasing the chance of T cell activation. Both B and T lymphocytes enter lymph nodes from circulating blood through specialized high endothelial venules found in the paracortex. ==Clinical significance==

Swelling Lymph node enlargement or swelling is known as lymphadenopathy. Swelling may be due to many causes, including infections, tumors, autoimmune disease, drug reactions, diseases such as amyloidosis and sarcoidosis, or because of lymphoma or leukemia. Depending on the cause, swelling may be painful, particularly if the expansion is rapid and due to an infection or inflammation. Lymph node enlargement may be localized to an area, which might suggest a local source of infection or a tumour in that area that has spread to the lymph node. It may also be generalized, which might suggest infection, connective tissue or autoimmune disease, or a malignancy of blood cells such as a lymphoma or leukemia. Rarely, depending on location, lymph node enlargement may cause problems such as difficulty breathing, or compression of a blood vessel (for example, superior vena cava obstruction). Enlarged lymph nodes might be felt as part of a medical examination, or found on medical imaging. Features of the medical history may point to the cause, such as the speed of onset of swelling, pain, and other constitutional symptoms such as fevers or weight loss. For example, a tumour of the breast may result in swelling of the lymph nodes under the arms and weight loss and night sweats may suggest a malignancy such as lymphoma. In addition to a medical exam by a medical practitioner, medical tests may include blood tests and scans may be needed to further examine the cause. A biopsy of a lymph node may also be needed. Cancer of a mesenteric lymph node with adenocarcinoma Lymph nodes can be affected by both primary cancers of lymph tissue, and secondary cancers affecting other parts of the body. Primary cancers of lymph tissue are called lymphomas and include Hodgkin lymphoma and non-Hodgkin lymphoma. Cancer of lymph nodes can cause a wide range of symptoms from painless long-term slowly growing swelling to sudden, rapid enlargement over days or weeks, with symptoms depending on the grade of the tumour. Most lymphomas are tumours of B-cells. Lymphoma is managed by haematologists and oncologists. Local cancer in many parts of the body can cause lymph nodes to enlarge because of tumorous cells that have metastasised into the node. Lymph node involvement is often a key part in the diagnosis and treatment of cancer, acting as "sentinels" of local disease, incorporated into TNM staging and other cancer staging systems. As part of the investigations or workup for cancer, lymph nodes may be imaged or even surgically removed. If removed, the lymph node will be stained and examined under a microscope by a pathologist to determine if there is evidence of cells that appear cancerous (i.e. have metastasized into the node). The staging of the cancer, and therefore the treatment approach and prognosis, is predicated on the presence of node metastases. Lymphedema Lymphedema is the condition of swelling (edema) of tissue relating to insufficient clearance by the lymphatic system. It can be congenital as a result usually of undeveloped or absent lymph nodes, and is known as primary lymphedema. Lymphedema most commonly arises in the arms or legs, but can also occur in the chest wall, genitals, neck, and abdomen. Secondary lymphedema usually results from the removal of lymph nodes during breast cancer surgery or from other damaging treatments such as radiation. It can also be caused by some parasitic infections. Affected tissues are at a great risk of infection. Management of lymphedema may include advice to lose weight, exercise, keep the affected limb moist, and compress the affected area. Sometimes surgical management is also considered. == Similar lymphoid organs ==

The spleen and the tonsils are the larger secondary lymphoid organs that serve somewhat similar functions to lymph nodes, though the spleen filters blood cells rather than lymph. The tonsils are sometimes erroneously referred to as lymph nodes. Although the tonsils and lymph nodes do share certain characteristics, there are also many important differences between them, such as their location, structure and size. Furthermore, the tonsils filter tissue fluid whereas lymph nodes filter lymph. == See also ==