Lymphatic system
Lymphatic system | |
---|---|
Details | |
Identifiers | |
Latin | systema lymphoideum |
MeSH | D008208 |
TA98 | A13.0.00.000 |
TA2 | 5149 |
FMA | 7162 74594, 7162 |
Anatomical terminology |
The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphoid organs, lymphatic tissue and lymph.[1][2] Lymph is a clear fluid carried by the lymphatic vessels back to the heart for re-circulation. The Latin word for lymph, lympha, refers to the deity of fresh water, "Lympha".[3]
Unlike the circulatory system that is a
The other main function is that of immune defense. Lymph is very similar to blood plasma, in that it contains waste products and
Fluid from circulating blood leaks into the tissues of the body by capillary action, carrying nutrients to the cells. The fluid bathes the tissues as interstitial fluid, collecting waste products, bacteria, and damaged cells, and then drains as lymph into the lymphatic capillaries and lymphatic vessels. These vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials such as bacteria and damaged cells. Lymph then passes into much larger lymph vessels known as lymph ducts. The right lymphatic duct drains the right side of the region and the much larger left lymphatic duct, known as the thoracic duct, drains the left side of the body. The ducts empty into the subclavian veins to return to the blood circulation. Lymph is moved through the system by muscle contractions.[9] In some vertebrates, a lymph heart is present that pumps the lymph to the veins.[9][10]
The lymphatic system was first described in the 17th century independently by Olaus Rudbeck and Thomas Bartholin.[11]
Structure
The lymphatic system consists of a conducting network of lymphatic vessels, lymphoid organs, lymphoid tissues, and the circulating lymph.[1]
Primary lymphoid organs
The primary (or central) lymphoid organs generate lymphocytes from immature progenitor cells. The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues.
Bone marrow
Bone marrow is responsible for both the creation of T cell precursors and the production and maturation of B cells, which are important cell types of the immune system. From the bone marrow, B cells immediately join the circulatory system and travel to secondary lymphoid organs in search of pathogens. T cells, on the other hand, travel from the bone marrow to the thymus, where they develop further and mature. Mature T cells then join B cells in search of pathogens. The other 95% of T cells begin a process of apoptosis, a form of programmed cell death.
Thymus
The thymus increases in size from birth in response to postnatal antigen stimulation. It is most active during the neonatal and pre-adolescent periods. The thymus is located between the inferior neck and the superior thorax. At puberty, by the early teens, the thymus begins to atrophy and regress, with adipose tissue mostly replacing the thymic stroma. However, residual T cell lymphopoiesis continues throughout adult life, providing some immune response. The thymus is where the T lymphocytes mature and become immunocompetent. The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. In most species, the thymus consists of lobules divided by septa which are made up of epithelium which is often considered an epithelial organ. T cells mature from thymocytes, proliferate, and undergo a selection process in the thymic cortex before entering the medulla to interact with epithelial cells.
Research on
The thymus provides an inductive environment for the development of T cells from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance. However, the thymus is not where the infection is fought, as the T cells have yet to become immunocompetent.
Secondary lymphoid organs
The secondary (or peripheral) lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response.[13] The secondary lymphoid organs are the sites of lymphocyte activation by antigens.[14] Activation leads to clonal expansion, and affinity maturation. Mature lymphocytes recirculate between the blood and the secondary lymphoid organs until they encounter their specific antigen.
Spleen
The main functions of the spleen are:
- to produce immune cells to fight antigens
- to remove particulate matter and aged blood cells, mainly red blood cells
- to produce blood cells during fetal life.
The spleen synthesizes
Research on
Like the
In the human until the fifth month of
Lymph nodes
A
The substance of a lymph node consists of lymphoid follicles in an outer portion called the
A lymph follicle is a dense collection of lymphocytes, the number, size, and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly when encountering a foreign antigen. The selection of
Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT).
In the
Tertiary lymphoid organs
Tertiary lymphoid organs (TLOs) are abnormal lymph node-like structures that form in peripheral tissues at sites of
TLOs are thought to play an important role in the immune response to cancer and to have possible implications in immunotherapy. They have been observed in a number of cancer types such as melanoma, non-small cell lung cancer and colorectal cancer (reviewed in [26]) as well as glioma.[27] Patients with TLOs in the vicinity of their tumors tend to have a better prognosis,[28][29] although the opposite is true for certain cancers.[30] TLOs that contain an active germinal center tend to have a better prognosis than those with TLOs without a germinal center.[28][29] The reason that these patients tend to live longer is thought to be the immune response against the tumor, which is mediated by the TLOs. TLOs may also promote an anti-tumor response when patients are treated with immunotherapy.[31] TLOs have been referred to in many different ways, including as tertiary lymphoid structures (TLS) and ectopic lymphoid structures (ELS). When associated with colorectal cancer, they are often referred to as a Crohn's-like lymphoid reaction.[28]
Other lymphoid tissue
Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against
The central nervous system also has lymphatic vessels. The search for T cell gateways into and out of the meninges uncovered functional meningeal lymphatic vessels lining the dural sinuses, anatomically integrated into the membrane surrounding the brain.[33]
Lymphatic vessels
The
The tissues of the lymphatic system are responsible for maintaining the balance of the body fluids. Its network of capillaries and collecting lymphatic vessels work to efficiently drain and transport extravasated fluid, along with proteins and antigens, back to the circulatory system. Numerous intraluminal valves in the vessels ensure a unidirectional flow of lymph without reflux.[35] Two valve systems, a primary and a secondary valve system, are used to achieve this unidirectional flow.[36] The capillaries are blind-ended, and the valves at the ends of capillaries use specialised junctions together with anchoring filaments to allow a unidirectional flow to the primary vessels. When interstitial fluid increases, it causes swelling that stretches collagen fibers anchored to adjacent connective tissue, in turn opening the unidirectional valves at the ends of these capillaries, facilitating the entry and subsequent drainage of excess lymph fluid. The collecting lymphatics, however, act to propel the lymph by the combined actions of the intraluminal valves and lymphatic muscle cells.[37]
Development
Lymphatic tissues begin to develop by the end of the fifth week of embryonic development.
Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.
The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins.
From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck, and head.
Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.
The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach.
The thymus arises as an outgrowth of the third pharyngeal pouch.
Function
The lymphatic system has multiple interrelated functions:[38][39][40][41][42][43][44]
- It is responsible for the removal of interstitial fluidfrom tissues
- It absorbs and transports fatty acids and fats as chyle from the digestive system
- It transports white blood cellsto and from the lymph nodes into the bones
- The lymph transports dendritic cells, to the lymph nodes where an immune response is stimulated.
Fat absorption
Immune function
The lymphatic system plays a major role in the body's immune system, as the primary site for cells relating to
Cells in the lymphatic system react to
When an antigen is recognized, an immunological cascade begins involving the activation and recruitment of more and more cells, the production of
Clinical significance
The study of lymphatic drainage of various organs is important in the diagnosis, prognosis, and treatment of cancer. The lymphatic system, because of its closeness to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumours.
Enlarged lymph nodes
Lymphedema
Lymphedema is the swelling caused by the accumulation of lymph, which may occur if the lymphatic system is damaged or has malformations. It usually affects limbs, though the face, neck and abdomen may also be affected. In an extreme state, called elephantiasis, the edema progresses to the extent that the skin becomes thick with an appearance similar to the skin on elephant limbs.[46]
Causes are unknown in most cases, but sometimes there is a previous history of severe infection, usually caused by a parasitic disease, such as lymphatic filariasis.
Lymphedema can also occur after
Cancer
Lymphoma is generally considered as either
Other
History
In the mid 16th century,
The idea that blood recirculates through the body rather than being produced anew by the liver and the heart was first accepted as a result of works of William Harvey—a work he published in 1628. In 1652, Olaus Rudbeck (1630–1702) discovered certain transparent vessels in the liver that contained clear fluid (and not white), and thus named them hepatico-aqueous vessels. He also learned that they emptied into the thoracic duct and that they had valves.[51] He announced his findings in the court of Queen Christina of Sweden, but did not publish his findings for a year,[53] and in the interim similar findings were published by Thomas Bartholin, who additionally published that such vessels are present everywhere in the body, not just in the liver. He is also the one to have named them "lymphatic vessels."[51] This had resulted in a bitter dispute between one of Bartholin's pupils, Martin Bogdan,[54] and Rudbeck, whom he accused of plagiarism.[53]
Galen's ideas prevailed in medicine until the 17th century. It was thought that blood was produced by the liver from chyle contaminated with ailments by the intestine and stomach, to which various spirits were added by other organs, and that this blood was consumed by all the organs of the body. This theory required that the blood be consumed and produced many times over. Even in the 17th century, his ideas were defended by some physicians.[55][56][57]
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"Claude Galien". Lithograph by Pierre Roche Vigneron. (Paris: Lith de Gregoire et Deneux, c. 1865)
-
Portrait of Eustachius
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Olaus Rudbeck in 1696.
Etymology
Lymph originates in the Classical Latin word lympha "water",[59] which is also the source of the English word limpid. The spelling with y and ph was influenced by folk etymology with Greek νύμϕη (nýmphē) "nymph".[60]
The adjective used for the lymph-transporting system is lymphatic. The adjective used for the tissues where lymphocytes are formed is lymphoid. Lymphatic comes from the Latin word lymphaticus, meaning "connected to water."
See also
- List of lymphatic vessels of the human body.
- American Society of Lymphology
- Glymphatic system and Meningeal lymphatic vessels - equivalent for the central nervous system
- Innate lymphoid cells
- Lymphangiogenesis
- Lymphangion
- Mononuclear phagocyte system
- Waldemar Olszewski – discovered fundamental processes in human tissues connected with function of the lymphatic system
- Trogocytosis
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we discovered functional lymphatic vessels lining the dural sinuses. These structures express all of the molecular hallmarks of lymphatic endothelial cells, are able to carry both fluid and immune cells from the cerebrospinal fluid, and are connected to the deep cervical lymph nodes. The unique location of these vessels may have impeded their discovery to date, thereby contributing to the long-held concept of the absence of lymphatic vasculature in the central nervous system. The discovery of the central nervous system lymphatic system may call for a reassessment of basic assumptions in neuroimmunology and sheds new light on the aetiology of neuroinflammatory and neurodegenerative diseases associated with immune system dysfunction.
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{{cite book}}
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External links
- Lymphatic System
- Lymphatic System Overview (innerbody.com)