||It has been suggested that Capsule of lymph node be merged into this article. (Discuss) Proposed since April 2014.|
Schematic diagram of lymph node showing the flow of lymph through the lymph sinuses. Note: Outflowing lymph has more lymphocytes
A lymph node or lymph gland is an oval-shaped organ of the lymphatic system, distributed widely throughout the body including the armpit and stomach and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immunity cells. Lymph nodes act as filters or traps for foreign particles and are important in the proper functioning of the immune system. They are packed tightly with the white blood cells called lymphocytes and macrophages.
Lymph nodes also have clinical significance. They become inflamed or enlarged in various infections and diseases which may range from trivial, such as a throat infection, to life-threatening such as cancers. In the latter, the condition of lymph nodes is so significant that it is used for cancer staging, which decides the treatment to be employed, and for determining the prognosis. When swollen, inflamed or enlarged, lymph nodes can be hard, firm or tender.
Lymph nodes are not part of the body's system for handling illnesses stemming, for example, from environmental toxicity or personal injury—those tasks are handled by the liver and kidneys in the former and the muscular-skeletal system in the latter. The lymphatic system is structured to address biological infection by other organisms as well as biological breakdowns/ errors within the same body (i.e., cancer). "Illness" and "disease" may be caused by any number of events including biological, chemical, physical, and psychological sources among others. A response to illness that includes inflammation of the lymph nodes specifically indicates a biological factor.
Lymph nodes can also be diagnosed by biopsy whenever they are inflamed. Certain diseases affect lymph nodes with characteristic consistency and location.
The lymph node is surrounded by a fibrous capsule, and inside the lymph node the fibrous capsule extends to form trabeculae. The substance of the lymph node is divided into the outer cortex and the inner medulla surrounded by the cortex all around except at the hilum, where the medulla comes in direct contact with the surface.
Thin reticular fibers, elastin and reticular fibers form a supporting meshwork called reticular network (RN) inside the node, within which the white blood cells (WBCs), the most prominent ones being lymphocytes, are tightly packed as follicles in the cortex. Elsewhere, there are only occasional WBCs. The RN not only provides the structural support, but also the surface for adhesion of the dendritic cells, macrophages and lymphocytes. It 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.
A lymph sinus is a channel within the lymph node lined by the endothelial cells along with fibroblastic reticular cells and allows smooth flow of lymph through them. Thus, the subcapsular sinus is a sinus immediately deep to the capsule, and its endothelium is continuous with that of the afferent lymph vessel. It is also continuous with similar sinuses flanking the trabeculae and within the cortex (cortical sinuses). The cortical sinuses and that flanking the trabeculae drain into the medullary sinuses, from where the lymph flows into the efferent lymph vessel.
Multiple afferent lymph vessels that branch and network extensively within the capsule bring lymph into the lymph node. This lymph enters the subcapsular sinus. The innermost lining of the afferent lymph vessels is continuous with the cells lining the lymph sinuses. The lymph gets slowly filtered through the substance of the lymph node and ultimately reaches the medulla. In its course it encounters the lymphocytes and may lead to their activation as a part of adaptive immune response.
The cortex of the lymph node is the peripheral portion underneath the capsule. In the cortex, the subcapsular sinus drains to trabecular sinuses, and then the lymph flows into the medullary sinuses.
The outer cortex consists mainly of the B cells arranged as follicles, which may develop a germinal center when challenged with an antigen, and the deeper cortex mainly consisting of the T cells. There is a zone known as the subcortical zone where T-cells (or cells that are mainly red) mainly interact with dendritic cells, and where the reticular network is dense. The predominant site within the lymph nodes which contain T cells & accessory cells is also known as paracortex (reticular network).
There are two named structures in the medulla:
- The medullary cords are cords of lymphatic tissue, and include plasma cells, macrophages, and B cells
- The medullary sinuses (or sinusoids) are vessel-like spaces separating the medullary cords. Lymph flows into the medullary sinuses from cortical sinuses, and into efferent lymphatic vessels. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells.
The medulla contains large blood vessels, sinuses and medullary cords that contain plasma cells secreting antibody.
Shape and size
Human lymph nodes are bean-shaped and range in size from a few millimeters to about 1–2 cm long. They may become enlarged due to a tumor or infection, or inflamed due to leukemia. Lymphocytes, types of white blood cells, are located within honeycomb structures of the lymph nodes. Lymph nodes are enlarged when the body is infected, primarily because there is an elevated rate of trafficking of lymphocytes into the node from the blood, exceeding the rate of outflow from the node, and secondarily as a result of the activation and proliferation of antigen-specific T and B cells (clonal expansion). In some cases, they may feel enlarged because of a previous infection; although one may be healthy, one may still feel them residually enlarged.
The lymph fluid inside the lymph nodes contains lymphocytes, a type of white blood cell, which continuously recirculates through the lymph nodes and the bloodstream. Molecules found on bacteria cell walls or chemical substances secreted from bacteria, called antigens, may be taken up by dedicated antigen presenting cells such as dendritic cells into the lymph system and then into lymph nodes. In response to the antigens, the lymphocytes in the lymph node make antibodies which will go out of the lymph node into circulation, seek, and target the pathogens producing the antigens by targeting them for destruction by other cells and complement. Other immune system cells will be made to fight the infection and "sent" to the lymph nodes.[clarification needed] The increased numbers of immune system cells fighting the infection will make the node expand and become "swollen."
Lymph circulates 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 towards either a more central lymph node or ultimately for drainage into a central venous subclavian blood vessel, most via the postcapillary venules, and cross its wall by the process of diapedesis.
- The B cells migrate to the nodular cortex and medulla.
- The T cells migrate to the deep cortex ("paracortex").
When a lymphocyte recognizes an antigen, B cells become activated and migrate to germinal centers (by definition, a "secondary nodule" has a germinal center, while a "primary nodule" does not). 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.
Lymphadenopathy is a term meaning "disease of the lymph nodes." It is, however, almost synonymously used with "swollen or enlarged lymph nodes." In this case, the lymph nodes are detectable by touch (palpable); this is a sign of various infections and diseases.
Lymphodoema is a fairly widespread condition of the lymphatic system resulting in localised fluid retention and tissue swelling. Affected tissues are at risk of infection. Primary lymphodoema generally results from poorly developed or missing lymph nodes. Secondary lymphodoema results mostly from the dissection of lymph nodes during breast cancer surgery and also from other treatments usually involving radiation.
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