Mast cell degranulation is a central component of all allergic diseases, and the clinical and pathologic manifestations of the diseases depend on the tissues in which the mast cell mediators have effects as well as the chronicity of the resulting inflammatory process. Atopic individuals may have one or more manifestations of allergic disease. The most common forms of these diseases are allergic rhinitis (hay fever), bronchial asthma, atopic dermatitis (eczema), and food allergies. The clinical and pathologic features of allergic reactions vary with the anatomic site of the reaction, for several reasons. The point of contact with the allergen determines the organs or tissues that are involved. For example, inhaled antigens cause rhinitis or asthma, ingested antigens often cause vomiting and diarrhea, and injected antigens cause systemic effects on the circulation. The concentration of mast cells in various target organs influences the severity of responses. Mast cells are particularly abundant in the skin and the mucosa of the respiratory and gastrointestinal tracts, and these tissues frequently suffer the most injury in immediate hypersensitivity reactions. The local mast cell phenotype may influence the characteristics of the immediate hyper-sensitivity reaction. For example, connective tissue mast cells produce abundant histamine and are responsible for wheal and flare reactions in the skin. In the following section, we discuss the major features of allergic diseases manifested in different tissues.
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