An allergy is a hypersensitivity disorder of the immune system in which exposure, either in the form of ingestion, inhalation or direct contact, to an antigen considered harmless under normal circumstances initiates the production of immune cells expressing sensitivity to that specific allergen, which upon subsequent instances of exposure results in an adverse reaction. The occurrence of an allergic reaction is a distinct one in that following an initial instance of exposure, in which the mast cells and basophils of the immune system are sensitized to the allergen by the immunoglobulin E antibody, any subsequent exposure can result in the excessive activation of these immune cells leading to the secretion of histamine, along with other inflammatory factors. Histamine, which is commonly considered primarily responsible for the majority allergy related symptoms, is released from the activated mast cells and basophils during degranulation, a process in which the contents of cellular granules are voided into surrounding tissues. Symptoms generally associated with an allergic reaction include vascular dilation, mucous secretion, smooth muscle contraction, nerve stimulation, and inflammation. Largely dependent upon the specific sensitivity of an individual to a particular allergen, as well as the degree and mode of exposure, the severity of an allergic reaction can vary drastically from mild, typified by seasonal allergies such as hay fever, to potentially life-threatening, in some cases of exposure to environmental, dietary or pharmacologic allergens, and can result in an unpredictable inflammatory response of either localized or generalized proportions.
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