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Basophils and mast cells share biochemical and functional characteristics, but they are not identical.* In humans, basophils are the least frequent of the three granulocytes, typically accounting for less than 0.5 percent of blood leukocytes. Basophils circulate as mature cells and can be recruited into tissues, particularly at sites of immunologic or inflammatory responses, but they ordinarily do not reside in tissues. By contrast, mast cells typically are derived from blood precursors that lack many of the characteristic features of the mature cells and complete their maturation in the tissues. The mature mast cells can reside in tissues for long periods of time. Mast cells are particularly abundant near blood vessels and nerves and in connective tissues beneath surfaces that are exposed to the external environment, such as the skin, gastrointestinal and urogenital tracts, and respiratory system. Tissue mast cell numbers can increase at sites of parasite infection or in association with certain chronic allergic diseases or other forms of pathology, by recruitment and local maturation of blood precursors and by proliferation of resident mast cells.

Mast cells and basophils express the high-affinity receptor for immunoglobulin (Ig) E (FcεRI) on their surface. Both cell types can be triggered to release potent mediators in response to activation via FcεRI, for example, when their cell-bound IgE recognizes bivalent or multivalent allergens. Accordingly, mast cells and basophils have long been regarded as important effector cells in asthma, hay fever, and other allergic disorders. The cells’ cytoplasmic granule-associated preformed mediators, including histamine and certain proteases, their lipid mediators (such as prostaglandin D2 and leukotriene C4), which are generated upon activation of the cells, and their cytokines, growth factors, and chemokines contribute to many of the characteristic signs and symptoms of these diseases. However, several lines of evidence indicate mast cells and basophils also contribute to protective host responses associated with IgE production, especially those directed against parasites. In mice, mast cells can enhance innate and acquired (IgE-dependent) defense against animal venoms and also can contribute to host defense in innate immune responses to certain bacterial infections. Mast cells and basophils also may express positive and negative immunoregulatory functions through cytokine production and other mechanisms.

Although a variety of systemic disorders are associated with changes in the numbers of blood basophils and many pathologic processes can be associated with changes in the numbers of tissue mast cells, patients with primary deficiencies in basophils appear to be exceedingly rare (if they exist at all). Patients with a primary deficiency of tissue mast cells have not been reported. By contrast, neoplastic processes can affect both of the lineages. Increased numbers of basophils may be present in association with myeloproliferative neoplasms and several forms of myeloid leukemia. Increased numbers of basophils, sometimes to levels of 20 to 90 percent of blood leukocytes, occur in virtually all patients with chronic myelogenous leukemia. The basophils associated with both chronic myelogenous and acute myeloid ...

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