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Mast cells derive their name from the obsolete theory that its characteristic granula contain nutrients for other tissues. Nowadays, they are considered part of the immune system. Mast cells are very similar to basophil granulocytes (a class of white blood cells) in blood; the similarities between mast cells and basophils has led many to speculate that mast cells are basophils that have "homed in" on tissues. However, current evidence suggests that they are generated by different precursor cells in the bone marrow. Nevertheless, both mast cells and basophils are thought to originate from bone marrow precursors expressing the CD34 molecule. The basophil leaves the bone marrow already mature while the mast cell circulates in an immature form, only maturing once in a tissue site. The tissue site an immature mast cell choses to settle in probably determines its precise characteristics.
Two types of mast cells are recognised, those from connective tissue and a distinct set of mucosal mast cells, the activities of the latter are dependent on T-cells.
Mast cells are present in the skin, mucosa of the lungThe lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. Its function is to exchange oxygen from air with carbon dioxide from blood. The process in which this happens is called " extes and digestive tract, as well as in the mouthThe mouth is the opening through which an animal takes in food. It is usually located in the head, but not always; the mouth of a planarium is in the middle of its belly. Most animals have a complete digestive system, with a mouth at one end and an anus a and noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration. In most mammals, it also houses the nosehairs, which catch airborne particles and prevent them from reaching the lungs..
Mast cells express a high-affinity receptor (FcεRI) for Immunoglobulin E (IgE), the least abundant member of the antibodies. IgE is, in turn, synthetised by B-cells (the antibody-producing cells of the immune system). IgE molecules, like all antibodies, are specific for one specific epitopeAn epitope is the part of a foreign organism (or its proteins) that is being recognised by the immune system and targeted by antibodies, cytotoxic T cells or both. Epitopes can be mapped using the ELISPOT or ELISA techniques. Immune system..
Mast cells remain inactive until an allergenAn allergen is any substance that causes an allergic reaction, most often when eaten or inhaled. No comprehensive list of allergens is possible, because sensitivities vary from one person to another and it is possible to be allergic to literally anything. binds to the cell. Allergens are generally proteins or polysaccharides. The allergen binds to the Fab part of the IgE molecules coated on the mast cell surface. It appears that binding to two IgE molecules is required to activate the mast cell; the steric changes lead to a slight disturbance to the cell membrane structure, causing a complex sequence of reactions inside the cell that lead to activation of the cell.
When activated, a mast cell rapidly releases its characteristic granules and various hormonal mediators into the interstitium. Other forms of activation besides IgE (such as by the complement system) have been described.
The molecules thus released into the intercellular environment include:
Histamine leads to the contraction of smooth muscle cells, activates endothelium and irritates nerve endings (leading to itching or pain). Cuteanous signs of histamine release are the "flare and wheal "-reaction. This occurs seconds after challenge of the mast cell by an allergen.
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