Mast cell, Wright stain
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An increase in tissue concentration of mast cells is characteristic of type I hypersensitivity (anaphylactic type, mediated by IgE). All mast cells have IgE receptors on their surfaces, and can become sensitized by binding IgE that is specific for a given antigen (allergen). When stimulated, they are an exceptionally rich source of histamine in the tissues, produce eicosanoids, and produce an eosinophil chemotactic factor. The fact that eosinophils are commonly found at the site of allergic reactions, such as asthma, is probably due to the fact that they were attracted there by the eosinophil chemotactic factor that is produced by mast cells.