Lymphatic system

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Lymphatic system
Blausen 0623 LymphaticSystem Female.png
Human lymphatic system
Details
Identifiers
Latin systema lymphoideum
MeSH D008208
TA A13.0.00.000
FMA 74594
Anatomical terminology

The lymphatic system is part of the vascular system and an important part of the immune system, comprising a large network of lymphatic vessels that carry a clear fluid called lymph (from Latin, lympha meaning "water" [1] ) directionally towards the heart. The lymphatic system was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. Unlike the circulatory system, the lymphatic system is not a closed system. The human circulatory system processes an average of 20 litres of blood per day through capillary filtration, which removes plasma while leaving the blood cells. Roughly 17 litres of the filtered plasma is reabsorbed directly into the blood vessels, while the remaining three litres remain in the interstitial fluid. One of the main functions of the lymph system is to provide an accessory return route to the blood for the surplus three litres. [2]

Immune system A biological system that protects an organism against disease

The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. In many species, the immune system can be classified into subsystems, such as the innate immune system versus the adaptive immune system, or humoral immunity versus cell-mediated immunity. In humans, the blood–brain barrier, blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the neuroimmune system, which protects the brain.

Lymphatic vessel any pathway for lymphiod fluids

The lymphatic vessels are thin-walled vessels (tubes) structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessels are lined by endothelial cells, and have a thin layer of smooth muscle, and adventitia that bind the lymph vessels to the surrounding tissue. Lymph vessels are devoted to the propulsion of the lymph from the lymph capillaries, which are mainly concerned with absorption of interstitial fluid from the tissues. Lymph capillaries are slightly larger than their counterpart capillaries of the vascular system. Lymph vessels that carry lymph to a lymph node are called afferent lymph vessels, and those that carry it from a lymph node are called efferent lymph vessels, from where the lymph may travel to another lymph node, may be returned to a vein, or may travel to a larger lymph duct. Lymph ducts drain the lymph into one of the subclavian veins and thus return it to general circulation.

Water chemical compound

Water is a transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's streams, lakes, and oceans, and the fluids of most living organisms. It is vital for all known forms of life, even though it provides no calories or organic nutrients. Its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient temperature and pressure. It forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor. Water moves continually through the water cycle of evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea.

Contents

The other main function is that of defense in the immune system. Lymph is very similar to blood plasma: it contains lymphocytes. It also contains waste products and cellular debris together with bacteria and proteins. Associated organs composed of lymphoid tissue are the sites of lymphocyte production. Lymphocytes are concentrated in the lymph nodes. The spleen and the thymus are also lymphoid organs of the immune system. The tonsils are lymphoid organs that are also associated with the digestive system. Lymphoid tissues contain lymphocytes, and also contain other types of cells for support. [3] The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which also includes the bone marrow, and the lymphoid tissue associated with the digestive system. [4]

Lymphocyte Subtype of white blood cell

A lymphocyte is one of the subtypes of a white blood cell in a vertebrate's immune system. Lymphocytes include natural killer cells, T cells, and B cells. They are the main type of cell found in lymph, which prompted the name "lymphocyte".

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Protein biological molecule consisting of chains of amino acid residues

Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three-dimensional structure that determines its activity.

The blood does not come into direct contact with the parenchymal cells and tissues in the body (except in case of an injury causing rupture of one or more blood vessels), but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g., the contractions of skeletal muscles), or by lymph hearts in some animals. The organization of lymph nodes and drainage follows the organization of the body into external and internal regions; therefore, the lymphatic drainage of the head, limbs, and body cavity walls follows an external route, and the lymphatic drainage of the thorax, abdomen, and pelvic cavities follows an internal route. [5] Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins, near their junction with the internal jugular veins.

Parenchyma

Parenchyma is the bulk of a substance. In animals, a parenchyma comprises the functional parts of an organ and in plants parenchyma is the ground tissue of nonwoody structures.

Cell (biology) The basic structural and functional unit of all organisms; the smallest unit of life.

The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.

Tissue (biology) An ensemble of similar cells and their matrix with similar origin and function

In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

Structure

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Lymphatic system

The lymphatic system consists of lymphatic organs, a conducting network of lymphatic vessels, and the circulating lymph.

The primary or central lymphoid organs generate lymphocytes from immature progenitor cells.

Progenitor cell biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its "target" cell; can divide only a limited number of times

A progenitor cell is a biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its "target" cell. The most important difference between stem cells and progenitor cells is that stem cells can replicate indefinitely, whereas progenitor cells can divide only a limited number of times. Controversy about the exact definition remains and the concept is still evolving.

The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues. Bone marrow is responsible for both the creation of T cells and the production and maturation of B cells. 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. Mature T cells 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 organ of the immune system

The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. Histologically, each lobe of the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule. The cortex and medulla play different roles in the development of T cells. Cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin. Developing T cells are referred to as thymocytes and are of hematopoietic origin. Stromal cells include epithelial cells of the thymic cortex and medulla, and dendritic cells.

Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production or hematopoiesis. It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. In adult humans, bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis. On average, bone marrow constitutes 4% of the total body mass of humans; in an adult having 65 kilograms of mass, bone marrow typically accounts for approximately 2.6 kilograms (5.7 lb).

Clonal selection

Clonal selection theory is a scientific theory in immunology that explains the functions of cells (lymphocytes) of the immune system in response to specific antigens invading the body. The concept was introduced by the Australian doctor Frank Macfarlane Burnet in 1957, in an attempt to explain the formation of a diversity of antibodies during initiation of the immune response. The theory has become the widely accepted model for how the immune system responds to infection and how certain types of B and T lymphocytes are selected for destruction of specific antigens.

Secondary or peripheral lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigens. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.

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 gastrointestinal wall the appendix has mucosa resembling that of the colon, but here it is heavily infiltrated with lymphocytes.

Tertiary lymphoid organs (TLO) are abnormal lymph node–like structures that form in peripheral tissues at sites of chronic inflammation, such as chronic infection, transplanted organs undergoing graft rejection, some cancers, and autoimmune and autoimmune-related diseases. [6] TLOs are regulated differently from the normal process whereby lymphoid tissues are formed during ontogeny, being dependent on cytokines and hematopoietic cells, but still drain interstitial fluid and transport lymphocytes in response to the same chemical messengers and gradients. [7] TLOs typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph. [8]

Lymphoid tissue

Thymus

The thymus is a primary lymphoid organ and the site of maturation for T cells, the lymphocytes of the adaptive immune system. The thymus increases in size from birth in response to postnatal antigen stimulation, then to puberty and regresses thereafter. [9] The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. [9] In most species, the thymus consists of lobules divided by septa which are made up of epithelium and is therefore an epithelial organ. T cells mature from thymocytes, proliferate and undergo selection process in the thymic cortex before entering the medulla to interact with epithelial cells.

The thymus provides an inductive environment for 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.

The thymus is largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced by adipose tissue. Nevertheless, residual T lymphopoiesis continues throughout adult life.

Spleen

The main functions of the spleen are:

  1. to produce immune cells to fight antigens
  2. to remove particulate matter and aged blood cells, mainly red blood cells.
  3. to produce blood cells during fetal life

The spleen synthesizes antibodies in its white pulp and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation. A study published in 2009 using mice found that the spleen contains, in its reserve, half of the body's monocytes within the red pulp. [10] These monocytes, upon moving to injured tissue (such as the heart), turn into dendritic cells and macrophages while promoting tissue healing. [10] [11] [12] The spleen is a center of activity of the mononuclear phagocyte system and can be considered analogous to a large lymph node, as its absence causes a predisposition to certain infections.

Like the thymus, the spleen has only efferent lymphatic vessels. Both the short gastric arteries and the splenic artery supply it with blood. [13]

The germinal centers are supplied by arterioles called penicilliary radicles. [14]

Up to the fifth month of prenatal development the spleen creates red blood cells. After birth the bone marrow is solely responsible for hematopoiesis. As a major lymphoid organ and a central player in the reticuloendothelial system, the spleen retains the ability to produce lymphocytes. The spleen stores red blood cells and lymphocytes. It can store enough blood cells to help in an emergency. Up to 25% of lymphocytes can be stored at any one time. [15]

Lymph nodes

A lymph node showing afferent and efferent lymphatic vessels Illu lymph node structure.png
A lymph node showing afferent and efferent lymphatic vessels

A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way back to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is then drained out by an efferent lymph vessel. There are between five and six hundred lymph nodes in the human body, many of which are grouped in clusters in different regions as in the underarm and abdominal areas. Lymph node clusters are commonly found at the base of limbs (groin, armpits) and in the neck, where lymph is collected from regions of the body likely to sustain pathogen contamination from injuries.

The substance of a lymph node consists of lymphoid follicles in an outer portion called the cortex. The inner portion of the node is called the medulla, which is surrounded by the cortex on all sides except for a portion known as the hilum. The hilum presents as a depression on the surface of the lymph node, causing the otherwise spherical lymph node to be bean-shaped or ovoid. The efferent lymph vessel directly emerges from the lymph node at the hilum. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.

The region of the lymph node called the paracortex immediately surrounds the medulla. Unlike the cortex, which has mostly immature T cells, or thymocytes, the paracortex has a mixture of immature and mature T cells. Lymphocytes enter the lymph nodes through specialised high endothelial venules found in the paracortex.

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 B cells, or B lymphocytes, occurs in the germinal centre of the lymph nodes.

Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla, inguinal region, and in association with the blood vessels of the intestines. [4]

Other lymphoid tissue

Regional lymph nodes Lymph node regions.svg
Regional lymph nodes

Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against infections and the spread of tumours. It consists of connective tissue formed of reticular fibers, with various types of leukocytes, (white blood cells), mostly lymphocytes enmeshed in it, through which the lymph passes. [16] Regions of the lymphoid tissue that are densely packed with lymphocytes are known as lymphoid follicles. Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue.

The central nervous system also has lymphatic vessels, as discovered by the University of Virginia Researchers. 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. [17]

Lymphatic vessels

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Lymph capillaries in the tissue spaces

The lymphatic vessels, also called lymph vessels, conduct lymph between different parts of the body. They include the tubular vessels of the lymph capillaries, and the larger collecting vessels–the right lymphatic duct and the thoracic duct (the left lymphatic duct). The lymph capillaries are mainly responsible for the absorption of interstitial fluid from the tissues, while lymph vessels propel the absorbed fluid forward into the larger collecting ducts, where it ultimately returns to the bloodstream via one of the subclavian veins. These vessels are also called the lymphatic channels or simply lymphatics. [18]

The lymphatics 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. [19] Two valve systems are used to achieve this one directional flow—a primary and a secondary valve system. [20] 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. The collecting lymphatics, however, act to propel the lymph by the combined actions of the intraluminal valves and lymphatic muscle cells. [21]

Development

Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. [22] 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. [22] From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. [22] 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 next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighbouring veins.

The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.

With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes.

The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. [22] The thymus arises as an outgrowth of the third pharyngeal pouch.

Function

The lymphatic system has multiple interrelated functions: [23]

Fat absorption

Nutrients in food are absorbed via intestinal vili (greatly enlargened in the picture) to blood and lymph. Long-chain fatty acids (and other lipids with similar fat solubility like some medicines) are absorbed to the lymph and move in it enveloped inside chylomicrons. They move via thoracic duct of the lymphatic system and finally enter the blood via the left subclavian vein thus bypassing the liver's first pass metabolism completely. Nutrient absorbtion to blood and lymph.png
Nutrients in food are absorbed via intestinal vili (greatly enlargened in the picture) to blood and lymph. Long-chain fatty acids (and other lipids with similar fat solubility like some medicines) are absorbed to the lymph and move in it enveloped inside chylomicrons. They move via thoracic duct of the lymphatic system and finally enter the blood via the left subclavian vein thus bypassing the liver's first pass metabolism completely.

Lymph vessels called lacteals are at the beginning of the gastrointestinal tract, predominantly in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain via the portal vein into the liver for processing, fats (lipids) are passed on to the lymphatic system to be transported to the blood circulation via the thoracic duct. (There are exceptions, for example medium-chain triglycerides are fatty acid esters of glycerol that passively diffuse from the GI tract to the portal system.) The enriched lymph originating in the lymphatics of the small intestine is called chyle. The nutrients that are released into the circulatory system are processed by the liver, having passed through the systemic circulation.

Immune function

The lymphatic system plays a major role in the body's immune system, as the primary site for cells relating to adaptive immune system including T-cells and B-cells. Cells in the lymphatic system react to antigens presented or found by the cells directly or by other dendritic cells. When an antigen is recognized, an immunological cascade begins involving the activation and recruitment of more and more cells, the production of antibodies and cytokines and the recruitment of other immunological cells such as macrophages.

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

Lymphadenopathy refers to one or more enlarged lymph nodes. Small groups or individually enlarged lymph nodes are generally reactive in response to infection or inflammation. This is called local lymphadenopathy. When many lymph nodes in different areas of the body are involved, this is called generalised lymphadenopathy. Generalised lymphadenopathy may be caused by infections such as infectious mononucleosis, tuberculosis and HIV, connective tissue diseases such as SLE and rheumatoid arthritis, and cancers, including both cancers of tissue within lymph nodes, discussed below, and metastasis of cancerous cells from other parts of the body, that have arrived via the lymphatic system. [24]

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. [25]

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.

Lymphangiomatosis is a disease involving multiple cysts or lesions formed from lymphatic vessels.[ relevant to this paragraph? ]

Lymphedema can also occur after surgical removal of lymph nodes in the armpit (causing the arm to swell due to poor lymphatic drainage) or groin (causing swelling of the leg). Conventional treatment is by manual lymphatic drainage and compression garments. Two drugs for the treatment of lymphedema are in clinical trials: Lymfactin [26] and Ubenimex/Bestatin.

There is no evidence to suggest that the effects of manual lymphatic drainage are permanent. [27]

Cancer

Reed-Sternberg cells. Reed-Sternberg lymphocyte nci-vol-7172-300.jpg
Reed–Sternberg cells.

Cancer of the lymphatic system can be primary or secondary. Lymphoma refers to cancer that arises from lymphatic tissue. Lymphoid leukaemias and lymphomas are now considered to be tumours of the same type of cell lineage. They are called "leukaemia" when in the blood or marrow and "lymphoma" when in lymphatic tissue. They are grouped together under the name "lymphoid malignancy". [28]

Lymphoma is generally considered as either Hodgkin lymphoma or non-Hodgkin lymphoma. Hodgkin lymphoma is characterised by a particular type of cell, called a Reed–Sternberg cell, visible under microscope. It is associated with past infection with the Epstein-Barr Virus, and generally causes a painless "rubbery" lymphadenopathy. It is staged, using Ann Arbor staging. Chemotherapy generally involves the ABVD and may also involve radiotherapy. [24] Non-Hodgkin lymphoma is a cancer characterised by increased proliferation of B-cells or T-cells, generally occurs in an older age group than Hodgkin lymphoma. It is treated according to whether it is high-grade or low-grade, and carries a poorer prognosis than Hodgkin lymphoma. [24]

Lymphangiosarcoma is a malignant soft tissue tumour, whereas lymphangioma is a benign tumour occurring frequently in association with Turner syndrome. Lymphangioleiomyomatosis is a benign tumour of the smooth muscles of the lymphatics that occurs in the lungs.

Lymphoid leukaemia is another form of cancer where the host is devoid of different lymphatic cells.

Other

History

Hippocrates, in the 5th century BC, was one of the first people to mention the lymphatic system. In his work On Joints, he briefly mentioned the lymph nodes in one sentence. Rufus of Ephesus, a Roman physician, identified the axillary, inguinal and mesenteric lymph nodes as well as the thymus during the 1st to 2nd century AD. [29] The first mention of lymphatic vessels was in the 3rd century BC by Herophilos, a Greek anatomist living in Alexandria, who incorrectly concluded that the "absorptive veins of the lymphatics," by which he meant the lacteals (lymph vessels of the intestines), drained into the hepatic portal veins, and thus into the liver. [29] The findings of Ruphus and Herophilos were further propagated by the Greek physician Galen, who described the lacteals and mesenteric lymph nodes which he observed in his dissection of apes and pigs in the 2nd century AD. [29] [30]

In the mid 16th century, Gabriele Falloppio (discoverer of the fallopian tubes), described what is now known as the lacteals as "coursing over the intestines full of yellow matter." [29] In about 1563 Bartolomeo Eustachi, a professor of anatomy, described the thoracic duct in horses as vena alba thoracis. [29] The next breakthrough came when in 1622 a physician, Gaspare Aselli, identified lymphatic vessels of the intestines in dogs and termed them venue alba et lacteal, which is now known as simply the lacteals. The lacteals were termed the fourth kind of vessels (the other three being the artery, vein and nerve, which was then believed to be a type of vessel), and disproved Galen's assertion that chyle was carried by the veins. But, he still believed that the lacteals carried the chyle to the liver (as taught by Galen). [31] He also identified the thoracic duct but failed to notice its connection with the lacteals. [29] This connection was established by Jean Pecquet in 1651, who found a white fluid mixing with blood in a dog's heart. He suspected that fluid to be chyle as its flow increased when abdominal pressure was applied. He traced this fluid to the thoracic duct, which he then followed to a chyle-filled sac he called the chyli receptaculum, which is now known as the cisternae chyli; further investigations led him to find that lacteals' contents enter the venous system via the thoracic duct. [29] [31] Thus, it was proven convincingly that the lacteals did not terminate in the liver, thus disproving Galen's second idea: that the chyle flowed to the liver. [31] Johann Veslingius drew the earliest sketches of the lacteals in humans in 1647. [30]

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), a Swede, 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. [31] He announced his findings in the court of Queen Christina of Sweden, but did not publish his findings for a year, [32] 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." [31] This had resulted in a bitter dispute between one of Bartholin's pupils, Martin Bogdan, [33] and Rudbeck, whom he accused of plagiarism. [32]

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. [30]

Alexander Monro, of the University of Edinburgh Medical School, was the first to describe the function of the lymphatic system in detail. [34]

Etymology

Lymph originates in the Classical Latin word lympha "water", [35] 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". [36]

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

Related Research Articles

Lymph node organ of the lymphatic system

A lymph node or lymph gland is an ovoid or kidney-shaped organ of the lymphatic system, and of the adaptive immune system, that is widely present throughout the body. They are linked by the lymphatic vessels as a part of the circulatory system. Lymph nodes 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. Lymph nodes do not have a detoxification function, which is primarily dealt with by the liver and kidneys.

Extracellular fluid fluid that is outside the cells

Extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% of total body weight; women and the obese have a lower percentage than lean men. About two thirds of this is intracellular fluid within cells, and one third is the extracellular fluid. The main component of the extracellular fluid is the interstitial fluid that surrounds cells.

Lamina propria

The lamina propria is a thin layer of connective tissue that forms part of the moist linings known as mucous membranes or mucosa, which line various tubes in the body, such as the respiratory tract, the gastrointestinal tract, and the urogenital tract.

The interstitium is a contiguous fluid-filled space existing between a structural barrier, such as a cell wall or the skin, and internal structures, such as organs, including muscles and the circulatory system. The fluid in this space is called interstitial fluid, comprises water and solutes, and drains into the lymph system. The interstitial compartment is composed of connective and supporting tissues within the body – called the extracellular matrix – that are situated outside the blood and lymphatic vessels and the parenchyma of organs.

Gut-associated lymphoid tissue (GALT) is a component of the mucosa-associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut.

Myeloid tissue

Myeloid tissue, in the bone marrow sense of the word myeloid, is tissue of bone marrow, of bone marrow cell lineage, or resembling bone marrow, and myelogenous tissue is any tissue of, or arising from, bone marrow; in these senses the terms are usually used synonymously, as for example with chronic myeloid/myelogenous leukemia.

Germinal center

Germinal centers or germinal centres (GCs) are sites within secondary lymphoid organs – lymph nodes and the spleen where mature B cells proliferate, differentiate, and mutate their antibody genes, and switch the class of their antibodies during a normal immune response to an infection. These develop dynamically after the activation of follicular B cells by T-dependent antigen.

White pulp tissue in the spleen

White pulp is a histological designation for regions of the spleen, that encompasses approximately 25% of splenic tissue. White pulp consists entirely of lymphoid tissue.

High endothelial venules (HEV) are specialized post-capillary venous swellings characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules. HEVs enable lymphocytes circulating in the blood to directly enter a lymph node.

Tonsil

Tonsils are collections of lymphoid tissue facing into the aerodigestive tract. The set of lymphatic tissue known as Waldeyer's tonsillar ring includes the adenoid tonsil, two tubal tonsils, two palatine tonsils, and the lingual tonsil.

Lymph capillary

Lymph capillaries or lymphatic capillaries are tiny, thin-walled microvessels located in the spaces between cells which serve to drain and process extracellular fluid. Upon entering the lumen of a lymphatic capillary, the collected fluid and associated cells is known as lymph. Each lymphatic capillary carries lymph into a lymphatic vessel, which in turn connects to a lymph node. Lymph is ultimately returned to the venous circulation.

Lymphocyte homing receptors are cell adhesion molecules expressed on lymphocyte cell membranes that recognize addressins on target tissues. Lymphocyte homing refers to adhesion of the circulating lymphocytes in blood to specialized endothelial cells within lymphoid organs. These diverse tissue-specific adhesion molecules on lymphocytes and on endothelial cells contribute to the development of specialized immune responses.

Peripheral tolerance is the second branch of immunological tolerance, after central tolerance. It takes place in the immune periphery. Its main purpose is to ensure that self-reactive T and B cells which escaped central tolerance do not cause autoimmune disease.

Follicular B cells are a type of B cell that reside in primary and secondary lymphoid follicles of secondary and tertiary lymphoid organs, including spleen and lymph nodes. Antibody responses against proteins are believed to involve follicular B cell pathways in secondary lymphoid organs.

Lutzner cells

Lutzner cells were discovered by Marvin A. Lutzner, Lucien-Marie Pautrier, and Albert Sézary. These cells are also referred to as Pautrier’s abscess, Sézary’s cell, or Sézary-Lutzner cells. They are a form of T-lymphocytes that has been mutated This atypical form of T-lymphocytes contains T-cell receptors on the surface and is found in both the dermis and epidermis layers of the skin. Since Lutzner cells are a mutated form of T-lymphocytes, they develop in bone marrow and are transported to the thymus is order to mature. The production and maturation stages occur before the cell has developed a mutation. Lutzner cells can form cutaneous T-cell lymphoma, which is a form of skin cancer.

Lymph node stromal cells are essential to the structure and function of the lymph node. There are a number of different types of lymph node stromal cells which have a number of functions including: creating a tissue scaffold within lymph nodes for the support of hematopoietic cells; the release of small molecules that are chemical messengers that facilitate interactions between hematopoietic cells; the facilitation of the migration of hematopoietic cells; the presentation of antigens to immune cells at the initiation of the adaptive immune system; and the homeostasis of lymphocyte numbers. Stromal cells originate from multipotent mesenchymal stem cells.

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