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 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. 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; the other main function is that of defense in the immune system. Lymph is similar to blood plasma: it contains lymphocytes, it 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 lymphoid organs of the immune system. The tonsils are lymphoid organs that are associated with the digestive system. Lymphoid tissues contain lymphocytes, contain other types of cells for support; the system includes all the structures dedicated to the circulation and production of lymphocytes, which includes the bone marrow, the lymphoid tissue associated with the digestive system. The blood does not come into direct contact with the parenchymal cells and tissues in the body, 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, formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system; the lymph is 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, 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. 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; the lymphatic system consists of lymphatic organs, a conducting network of lymphatic vessels, the circulating lymph. 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 is responsible for both the creation of T cells and the production and maturation of B cells. From the bone marrow, B cells 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. Mature T cells join B cells in search of pathogens; the other 95 % of T cells begin a process of a form of programmed cell death.
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 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 to interact with the lymphocytes, it is exemplified by the lymph nodes, the lymphoid follicles in tonsils, Peyer's patches, adenoids, etc. that are associated with the mucosa-associated lymphoid tissue. In the gastrointestinal wall the appendix has mucosa resembling that of the colon, but here it is infiltrated with lymphocytes. Tertiary lymphoid organs 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, autoimmune and autoimmune-related diseases.
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. TLOs contains far fewer lymphocytes, assumes an immune role only when challenged with antigens that result in inflammation, it achieves this by importing the lymphocytes from lymph. 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 to puberty and regresses thereafter. 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 and is therefore an epithelial organ. T cells mature from
In epidemiology, a disease vector is any agent who carries and transmits an infectious pathogen into another living organism. Arthropods form a major group of pathogen vectors with mosquitoes, sand flies, fleas and mites transmitting a huge number of pathogens. Many such vectors are haematophagous, which feed on blood at all stages of their lives; when the insects blood feed, the pathogen enters the blood stream of the host. This can happen in different ways; the Anopheles mosquito, a vector for malaria and various arthropod-borne-viruses, inserts its delicate mouthpart under the skin and feeds on its host's blood. The parasites the mosquito carries are located in its salivary glands. Therefore, the parasites are transmitted directly into the host's blood stream. Pool feeders such as the sand fly and black fly, vectors for pathogens causing leishmaniasis and onchocerciasis will chew a well in the host's skin, forming a small pool of blood from which they feed. Leishmania parasites infect the host through the saliva of the sand fly.
Onchocerca force their own way out of the insect's head into the pool of blood. Triatomine bugs are responsible for the transmission of a trypanosome, Trypanosoma cruzi, which causes Chagas Disease; the Triatomine bugs defecate during feeding and the excrement contains the parasites which are accidentally smeared into the open wound by the host responding to pain and irritation from the bite. Some plants and fungi act as vectors for various pathogens. For example, the big-vein disease of lettuce was long thought to be caused by a member of the fungal division Chytridiomycota, namely Olpidium brassicae. However, the disease was shown to be viral, it transpired that the virus was transmitted by the zoospores of the fungus and survived in the resting spores. Since many other fungi in the Chytridiomycota have been shown to vector plant viruses. Many plant pests that damage important crops depend on other plants weeds, to harbour or vector them. In the case of Puccinia graminis for example and related genera act as alternate hosts in a cycle of infection of grain.
More directly, when they twine from one plant to another, parasitic plants such as Cuscuta and Cassytha have been shown to convey phytoplasmal and viral diseases between plants. The World Health Organization states that control and prevention of vector-borne diseases are emphasizing "Integrated Vector Management", an approach that looks at the links between health and environment, optimizing benefits to both. In April 2014, WHO launched a campaign called “Small bite, big threat” to educate people about vector-borne illnesses. WHO issued reports indicating that vector-borne illnesses affect poor people people living in areas that do not have adequate levels of sanitation, drinking water and housing. Several articles, recent to early 2014, warn that human activities are spreading vector-borne zoonotic diseases. Several articles were published in the medical journal The Lancet, discuss how rapid changes in land use, trade globalization, "social upheaval" are causing a resurgence in zoonotic disease across the world.
Examples of vector-borne zoonotic diseases include: Lyme disease Plague West Nile virusMany factors affect the incidence of vector-borne diseases. These factors include animals hosting the disease and people. Airborne disease Asymptomatic carrier Fomite Globalization and disease Insect vectors of human pathogens Insect vectors of plant pathogens VectorBase: genomic database of invertebrate vectors of human pathogens List of diseases caused by insects Natural reservoir Waterborne disease 2007 Yap Islands Zika virus outbreak "Better environmental management for control of dengue"; the Health and Environment Linkages Initiative. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. "Division of Vector-Borne Diseases". Fort Collins, Colorado: Centers for Disease Control and Prevention. Retrieved 7 April 2014. "Issue Brief Series: Vector-borne Diseases". Healthy Environments for Children Alliance. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. "Malaria control: the power of integrated action".
The Health and Environment Linkages Initiative. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. Pawan, J. L.. "Transmission of the Paralytic Rabies in Trinidad of the Vampire Bat: Desmodus rotundus murinus Wagner, 1840." Annual Tropical Medicine and Parasitol, 30, April 8, 1936:137–156. Pawan, J. L. "Rabies in the Vampire Bat of Trinidad with Special Reference to the Clinical Course and the Latency of Infection." Annals of Tropical Medicine and Parasitology. Vol. 30, No. 4. December 1936 Quammen, David. "Planet of the Ape. The New York Times. Retrieved 7 April 2014. "Vector-borne diseases". Articles about vector-borne disease. Vaccine News Daily. Chicago. WHO page on vector-borne diseasesBiological mosquito eradication in Monte Verde, Honduras
The Mayo Clinic is a nonprofit academic medical center based in Rochester, focused on integrated clinical practice and research. It employs more than 4,500 physicians and scientists, along with another 58,400 administrative and allied health staff; the practice specializes in treating difficult cases through tertiary destination medicine. It is home to the ranked Mayo Clinic Alix School of Medicine in addition to many of the largest, best regarded residency education programs in the United States, it has more than 3,000 full-time research personnel. William Worrall Mayo settled his family in Rochester in 1864 and opened a sole proprietorship medical practice that evolved under his sons and Charlie Mayo, into Mayo Clinic. Today, in addition to its flagship hospital in Rochester, Mayo Clinic has major campuses in Arizona and Florida; the Mayo Clinic Health System operates affiliated facilities throughout Minnesota and Iowa. Mayo Clinic is ranked number 1 in the United States on the 2018–2019 U. S. News & World Report Best Hospitals Honor Roll, maintaining a position at or near the top for more than 27 years.
It has been on the list of "100 Best Companies to Work For" published by Fortune magazine for fourteen consecutive years, has continued to achieve this ranking through 2017. In 1863, William Worrall Mayo came to Rochester, from Salford in Lancashire, England, as part of his appointment as an examining surgeon for the military draft board during the American Civil War; the city was to his liking, his wife and children joined him in early 1864. On January 27, 1864, William Worrall Mayo advertised in the Rochester City Post the opening of a private medical partnership "over the Union Drug Store on Third Street" with "all calls answered by day or night". Both of W. W. Mayo's sons, William James Mayo and Charles Horace Mayo grew up in Rochester and, when old enough, both attended medical school. William graduated in 1883 and joined his father's practice, with Charles joining after he completed his training in 1888. On August 21, 1883, a tornado struck Rochester, causing at least 37 deaths in the area and over 200 injuries.
One-third of the town was destroyed. The relief efforts began with a temporary hospital being established at Rommell's Hall, the doctors Mayo as well as other local doctors, were extensively involved in treating the injured who were brought there for help. Mother Alfred Moes and the Sisters of Saint Francis were called in to act as nurses despite having been trained as teachers and with little if any medical experience. After the crisis subsided, Moes approached W. W. Mayo about establishing a hospital in Rochester. Mayo agreed to work in the hospital and soon other local doctors agreed as well. On September 30, 1889, Saint Mary's Hospital was opened by the Sisters. W. W. Mayo, 70 years old, was one of the consulting physicians at the hospital, his two sons began performing surgeries at the hospital. In 1892, W. W. Mayo asked Augustus Stinchfield, whom he considered to be the best doctor in the area, to join the practice. After Stinchfield agreed, W. W. Mayo retired at the age of 73 and the practice continued to grow.
The founders of Mayo Clinic are the Mayo brothers Will and Charlie, Graham, Henry Plummer, Millet and Balfour. These early founders and partners shared in the profits of the private group practice, while other staff hired by the partners were salaried. W. W. Mayo died in 1911 and in 1919 the remaining founders, with the exception of Graham, created the Mayo Properties Association, their private practice became a not-for-profit entity; the founders gave the Clinic furnishings to this newly formed association. The integrated practice model developed by Plummer created a foundation for what would grow into Mayo Clinic; as the private practice grew, it required additional space. In 1914, the partners planned and built a new clinic building. Ellerbe Architects are the architect of record for the 1914 Mayo "Red" building, as well as for the 1922 Mayo Institute of Experimental Medicine, the 1927 Plummer building, the 1954 Mayo Clinic building, the 2002 Gonda building. In 1914, under the guidance of Henry Plummer, the new building allowed the integrated group medical practice concept to be expressed.
Many innovative medical systems and equipment were incorporated into the building design. Plummer worked with Frederic Maass, of Maass & McAndrew, to design and fabricate many of the building systems innovations like the steam sterilization rooms, metal surgical tools and equipment, pneumatic tube system, knee operated sinks, a state of the art HVAC system; the air exchange rate for the building was three minutes. One intriguing innovation was the Rookwood fountain in the main lobby, designed to clean and humidify air from the outside, it heated and humidified air in the winter, provided cool air in the summer. To fight infection, steam sterilizer rooms were designed to hold much of the operating rooms metal surgical furniture and equipment; these and other aseptic procedures helped bring the overall patient infection rates down. Until 1919 the Mayo Clinic was operated as a for-profit medical practice. In 1919, the Mayo brothers donated the clinic property and significant amounts of their wealth to develop the Mayo Properties Association.
The Association became the Mayo Clinic Foundation. The result of this was that the Mayo Clinic became a non-profit medical practice in 1920. In 1928, the Plummer Building was completed wit
Dormancy is a period in an organism's life cycle when growth and physical activity are temporarily stopped. This therefore helps an organism to conserve energy. Dormancy tends to be associated with environmental conditions. Organisms can synchronize entry to a dormant phase with their environment through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase before the onset of adverse conditions. For example and decreasing temperature are used by many plants to predict the onset of winter. Consequential dormancy occurs when organisms enter a dormant phase after adverse conditions have arisen; this is found in areas with an unpredictable climate. While sudden changes in conditions may lead to a high mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources. Hibernation is a mechanism used by many mammals to reduce energy expenditure and survive food shortage over the winter.
Hibernation may be consequential. An animal prepares for hibernation by building up a thick layer of body fat during late summer and autumn that will provide it with energy during the dormant period. During hibernation, the animal undergoes many physiological changes, including decreased heart rate and decreased body temperature. In addition to shivering, some hibernating animals produce body heat by non-shivering thermogenesis to avoid freezing. Non-shivering thermogenesis is a regulated process in which the proton gradient generated by electron transport in mitochondria is used to produce heat instead of ATP in brown adipose tissue. Animals that hibernate include bats, ground squirrels and other rodents, mouse lemurs, the European hedgehog and other insectivores and marsupials. Although hibernation is exclusively seen in mammals, some birds, such as the common poorwill, may hibernate. Diapause is a predictive strategy, predetermined by an animal's genotype. Diapause is common in insects, allowing them to suspend development between autumn and spring, in mammals such as the roe deer, in which a delay in attachment of the embryo to the uterine lining ensures that offspring are born in spring, when conditions are most favorable.
Aestivation spelled estivation, is an example of consequential dormancy in response to hot or dry conditions. It is common in invertebrates such as the garden snail and worm but occurs in other animals such as lungfish, desert tortoises, crocodiles. Brumation is an example of dormancy in reptiles, similar to hibernation, it differs from hibernation in the metabolic processes involved. Reptiles begin brumation in late autumn, they wake up to drink water and return to "sleep". They can go for months without food. Reptiles may eat more than usual before the brumation time but eat less or refuse food as the temperature drops. However, they do; the brumation period is anywhere from one to eight months depending on the air temperature and the size and health of the reptile. During the first year of life, many small reptiles do not brumate, but rather slow down and eat less often. Brumation is triggered by lack of heat and the decrease in the hours of daylight in winter, similar to hibernation. In plant physiology, dormancy is a period of arrested plant growth.
It is a survival strategy exhibited by many plant species, which enables them to survive in climates where part of the year is unsuitable for growth, such as winter or dry seasons. Many plant species that exhibit dormancy have a biological clock that tells them when to slow activity and to prepare soft tissues for a period of freezing temperatures or water shortage. On the other hand, dormancy can be triggered after a normal growing season by decreasing temperatures, shortened day length, and/or a reduction in rainfall. Chemical treatment on dormant plants has been proven to be an effective method to break dormancy in woody plants such as grapes, apples and kiwis. Hydrogen cyanamide stimulates cell division and growth in dormant plants, causing budbreak when the plant is on the edge of breaking dormancy. Slight injury of cells may play a role in the mechanism of action; the injury is thought to result in increased permeability of cellular membranes. The injury is associated with the inhibition of catalase, which in turn stimulates the pentose phosphate cycle.
Hydrogen cyanamide interacts with the cytokinin metabolic cycle, which results in triggering a new growth cycle. The images below show two widespread dormancy patterns amongst sympodially growing orchids: When a mature and viable seed under a favorable condition fails to germinate, it is said to be dormant. Seed dormancy is referred to as embryo dormancy or internal dormancy and is caused by endogenous characteristics of the embryo that prevent germination. Dormancy should not be confused with seed coat dormancy, external dormancy, or hardseededness, caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo, it is a physical barrier to germination, not a true form of dormancy. Temperate woody perennial plants require chilling temperatures to overcome winter dormancy; the effect of chilling temperatures depends on species and growth stage. In some species, rest can be broken within hours at
Salmonella is a genus of rod-shaped Gram-negative bacteria of the family Enterobacteriaceae. The two species of Salmonella are Salmonella Salmonella bongori. S. enterica is the type species and is further divided into six subspecies that include over 2,600 serotypes. Salmonella species are non-spore-forming, predominantly motile enterobacteria with cell diameters between about 0.7 and 1.5 µm, lengths from 2 to 5 µm, peritrichous flagella. They are chemotrophs, obtaining their energy from oxidation and reduction reactions using organic sources, they are facultative aerobes, capable of generating ATP with oxygen when it is available, or when oxygen is not available, using other electron acceptors or fermentation. S. enterica subspecies are found worldwide in the environment. S. bongori is restricted to cold-blooded animals reptiles. Salmonella species are intracellular pathogens. Nontyphoidal serotypes can be transferred from human-to-human, they invade only the gastrointestinal tract and cause salmonellosis, the symptoms of which can be resolved without antibiotics.
However, in sub-Saharan Africa, nontyphoidal Salmonella can be invasive and cause paratyphoid fever, which requires immediate treatment with antibiotics. Typhoidal serotypes can only be transferred from human-to-human, can cause food-borne infection, typhoid fever, paratyphoid fever. Typhoid fever is caused by Salmonella invading the bloodstream, or in addition spreads throughout the body, invades organs, secretes endotoxins; this can lead to life-threatening hypovolemic shock and septic shock, requires intensive care including antibiotics. The collapse of the Aztec society in Mesoamerica is linked to a catastrophic Salmonella outbreak, one of humanity's deadliest, that occurred after the Spanish conquest; the genus Salmonella is part of the family of Enterobacteriaceae. Its taxonomy has the potential to confuse; the genus comprises two species, S. bongori and S. enterica, the latter of, divided into six subspecies: S. e. enterica, S. e. salamae, S. e. arizonae, S. e. diarizonae, S. e. houtenae, S. e. indica.
The taxonomic group contains more than 2500 serotypes defined on the basis of the somatic O and flagellar H antigens. The full name of a serotype is given for example, Salmonella enterica subsp.. Enterica can be abbreviated to Salmonella Typhimurium. Further differentiation of strains to assist clinical and epidemiological investigation may be achieved by antibiotic sensitivity testing and by other molecular biology techniques such as pulsed-field gel electrophoresis, multilocus sequence typing, whole genome sequencing. Salmonellae have been clinically categorized as invasive or noninvasive based on host preference and disease manifestations in humans. Salmonella was first visualized in 1880 by Karl Eberth in the Peyer's patches and spleens of typhoid patients. Four years Georg Theodor Gaffky was able to grow the pathogen in pure culture. A year after that, medical research scientist Theobald Smith discovered what would be known as Salmonella enterica. At the time, Smith was working as a research laboratory assistant in the Veterinary Division of the United States Department of Agriculture.
The department was under the administration of a veterinary pathologist. Salmonella Choleraesuis was thought to be the causative agent of hog cholera, so Salmon and Smith named it "Hog-cholerabacillus"; the name Salmonella was not used until 1900, when Joseph Leon Lignières proposed that the pathogen discovered by Salmon's group be called Salmonella in his honor. Most subspecies of Salmonella produce hydrogen sulfide, which can be detected by growing them on media containing ferrous sulfate, such as is used in the triple sugar iron test. Most isolates exist in two phases, a motile phase I and a nonmotile phase II. Cultures that are nonmotile upon primary culture may be switched to the motile phase using a Craigie tube or ditch plate. RVS broth can be used to enrich for Salmonella species for detection in a clinical sample. Salmonella can be detected and subtyped using multiplex or real-time polymerase chain reactions from extracted Salmonella DNA. Mathematical models of Salmonella growth kinetics have been developed for chicken, pork and melons.
Salmonella reproduce asexually with a cell division interval of 40 minutes. Salmonella species lead predominantly host-associated lifestyles, but the bacteria were found to be able to persist in a bathroom setting for weeks following contamination, are isolated from water sources, which act as bacterial reservoirs and may help to facilitate transmission between hosts. Salmonella is notorious for its ability to survive desiccation and can persist for years in dry environments and foods; the bacteria are not destroyed by freezing. They perish after being heated to 60 °C for 12 min. To protect against Salmonella infection, heating food for at least 10 minutes to an internal temperature of 75 °C is recommended. Salmonella species can be found in the digestive tracts of humans and animals reptiles. Salmonella on the skin of reptiles or amphibians can be passed to people. Food and water can be contaminated with the bacteria if they come in contact with the feces of infected people
In biology, an organism is any individual entity that exhibits the properties of life. It is a synonym for "life form". Organisms are classified by taxonomy into specified groups such as the multicellular animals and fungi. All types of organisms are capable of reproduction and development, some degree of response to stimuli. Humans are multicellular animals composed of many trillions of cells which differentiate during development into specialized tissues and organs. An organism may be either a eukaryote. Prokaryotes are represented by two separate domains -- archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles. Fungi and plants are examples of kingdoms of organisms within the eukaryotes. Estimates on the number of Earth's current species range from 10 million to 14 million, of which only about 1.2 million have been documented. More than 99% of all species, amounting to over five billion species, that lived are estimated to be extinct.
In 2016, a set of 355 genes from the last universal common ancestor of all organisms was identified. The term "organism" first appeared in the English language in 1703 and took on its current definition by 1834, it is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kant's 1790 Critique of Judgment. An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, fungus or bacterium, capable of growth and reproduction". Many definitions exclude viruses and possible man-made non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction. A superorganism is an organism consisting of many individuals working together as a single functional or social unit. There has been controversy about the best way to define the organism and indeed about whether or not such a definition is necessary.
Several contributions are responses to the suggestion that the category of "organism" may well not be adequate in biology. Viruses are not considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism; this controversy is problematic because some cellular organisms are incapable of independent survival and live as obligatory intracellular parasites. Although viruses have a few enzymes and molecules characteristic of living organisms, they have no metabolism of their own; this rules out autonomous reproduction: they can only be passively replicated by the machinery of the host cell. In this sense, they are similar to inanimate matter. While viruses sustain no independent metabolism and thus are not classified as organisms, they do have their own genes, they do evolve by mechanisms similar to the evolutionary mechanisms of organisms; the most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly.
Some scientists argue. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible; this is not true for cells. If viruses did not exist, the direction of cellular evolution could be different, but cells would be able to evolve; as for the reproduction, viruses rely on hosts' machinery to replicate. The discovery of viral metagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong in the tree of life; the presence of these genes suggested. However, it was found that the genes coding for energy and protein metabolism have a cellular origin. Most these genes were acquired through horizontal gene transfer from viral hosts. Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival; the same laws that govern non-living chemistry govern the chemical processes of life.
It is the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes. Organisms owe their origin and many other internal functions to chemical phenomena the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles. Organisms are semi-closed chemical systems. Although they are individual units of life, they are not closed to the environment around them. To operate they take in and release energy. Autotrophs produce usable energy using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment; the primary chemical element in these compounds is carbon. The chemical properties of this element such as its grea
In biology and medicine, a host is an organism that harbours a parasitic, a mutualistic, or a commensalist guest, the guest being provided with nourishment and shelter. Examples include animals playing host to parasitic worms, cells harbouring pathogenic viruses, a bean plant hosting mutualistic nitrogen-fixing bacteria. More in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism; the host range is the collection of hosts. Symbiosis spans a wide variety of possible relationships between organisms, differing in their permanence and their effects on the two parties. If one of the partners in an association is much larger than the other, it is known as the host. In parasitism, the parasite benefits at the host's expense. In commensalism, the two live together without harming each other, while in mutualism, both parties benefit. Most parasites are only parasitic for part of their life cycle. By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions.
Some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate. A parasite may have a long term relationship with its host; the guest seeks out the host and obtains food or another service from it, but does not kill it. In contrast, a parasitoid spends a large part of its life within or on a single host causing the host's death, with some of the strategies involved verging on predation; the host is kept alive until the parasitoid is grown and ready to pass on to its next life stage. A guest's relationship with its host may be intermittent or temporary associated with multiple hosts, making the relationship equivalent to the herbivory of a wild-living animal. Another possibility is that the host–guest relationship may have no permanent physical contact, as in the brood parasitism of the cuckoo. Parasites follow a wide variety of evolutionary strategies, placing their hosts in an wide range of relationships. Parasitism implies host–parasite coevolution, including the maintenance of gene polymorphisms in the host, where there is a trade-off between the advantage of resistance to a parasite and a cost such as disease caused by the gene.
There are several kinds of host from a parasite's point of view. A definitive or primary host is one. An intermediate host is one. A virus is an obligate parasite, acting as a living thing only to the extent that when it is in a host cell, the machinery of that cell makes copies of the virus. A reservoir host can harbour a pathogen indefinitely with no ill effects, with important implications for disease control. A single reservoir host may be reinfected several times. A host of predilection is the one preferred by a parasite. An amplifying host is one in which the level of pathogen can become high enough that a vector such as a mosquito that feeds on it will become infectious. A secondary or intermediate host harbors a parasite only for a short transition period, during which some developmental stage is completed. For trypanosomes, the cause of sleeping sickness humans are the secondary host, while the tsetse fly is the primary host, given that it has been shown that reproduction occurs in the insect.
Tapeworms and other parasitic flatworms have complex lifecycles, in which specific developmental stages are completed in a sequence of several different hosts. It is not always easy or possible to identify which host is definitive and which secondary; as the life cycles of many parasites are not well understood, sometimes the subjectively more important organism is arbitrarily labelled as definitive, this designation may continue after it is found to be incorrect. For example, sludge worms are sometimes considered "intermediate hosts" for salmonid whirling disease though the myxosporean parasite reproduces sexually inside them. In trichinosis, a disease caused by roundworms, the host has reproductive adults in its digestive tract and immature juveniles in its muscles, is therefore both an intermediate and a definitive host. A paratenic host is similar to an intermediate host, except that it is not needed for the parasite's development cycle to progress. Paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers.
The trematode Alaria americana may serve as an example: the so-called mesocercarial stages of this parasite reside in tadpoles, which are eaten by the definitive canine host. The tadpoles are more preyed on by snakes, in which the mesocercariae may not undergo further development. However, the parasites may accumulate in the snake paratenic host and infect the definitive host once the snake is consumed by a canid; the nematode Skrjabingylus nasicola is another example, with slugs as the intermediate hosts and rodents as the paratenic hosts, mustelids as the definitive hosts. A dead-end or incidental host is an intermediate host that does not allow transmission to the definitive host, thereby preventing the parasite from completing its development. For example and horses are dead-end hosts for West Nile virus, whose life cycle is between culicine mosquitoes and birds. People and horses can become infected, but the level of virus in their blood does not become high enough to pass on the infection to mosquitoes that bite them.
Micropredation is an evolutionarily stable strategy within parasitism, in w