Allergic rhinitis, also called hay fever, is perhaps the most common allergic disease and is a consequence of immediate hypersensitivity reactions to common allergens such as plant pollen or house dust mites localized to the upper respiratory tract by inhalation. The pathologic and clinical manifestations include mucosal edema, leukocyte infiltration with abundant eosinophils, mucus secretion, coughing, sneezing, and difficulty in breathing. Allergic conjunctivitis with itchy eyes is commonly associated with the rhinitis. Focal protrusions of the nasal mucosa, called nasal polyps, filled with edema fluid and eosinophils may develop in patients who suffer frequent repetitive bouts of allergic rhinitis. Antihistamines are the most common drugs used to treat allergic rhinitis.
Food allergies are immediate hypersensitivity reactions to ingested foods that lead to the release of mediators from intestinal mucosal and submucosal mast cells. Clinical manifestations include enhanced peristalsis, increased fluid secretion from intestinal lining cells, and associated vomiting and diarrhea. Urticaria is often associated with allergic reactions to food, and systemic ana-phylaxis may occasionally ensue. Allergic reactions to many different types of food have been described, but some of the most common are peanuts and shellfish. Individuals may be so sensitive to these allergens that severe systemic reactions can occur in response to minute contaminants of the allergen introduced accidentally during food preparation.
Allergic reactions in the skin are manifested as urticaria and eczema. Urticaria, which is essentially an acute wheal and flare reaction induced by mast cell mediators, occurs in response to direct contact with the allergen or after an allergen enters the circulation through the intestinal tract or by injection. Because the reaction that ensues is mediated largely by histamine, antihistamines (H1 receptor antagonists) can block this response almost completely. The urticaria may persist for several hours, probably because antigen persists in the plasma. Chronic eczema (also called atopic dermatitis) is a common skin disorder that may be caused by a late-phase reaction to an allergen in the skin. In the cutaneous late-phase reaction, TNF, IL-4, and other cytokines, probably derived from TH2 cells and mast cells, act on the venular endothelial cells to promote inflammation. As may be expected for a cytokine-mediated response, the late-phase inflammatory reaction is not inhibited by antihistamines. It can be blocked by treatment with corticosteroids, which inhibit cytokine synthesis. Chronic contact sensitivity, which is a TH1-mediated DTH reaction, is also often called eczema. Another term used for immune-mediated skin disorders is atopic dermatitis; not all the cases are examples of atopy.
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