Allergic Reactions In The Skin

Urticaria is a short-lived, red, edematous lesion of the skin associated with itching. Nonimmunological and immunological mechanisms cause skin lesions. IgE-mediated immunological urticaria is more frequent in atopic subjects and is usually induced by penicillin or consumption of shellfish, soft fruits, and nuts (Anonymous, 1997). Nonimmunological urticaria can be elicited in healthy subjects by low-molecular-mass substances such as ammonium persulfate (Mahzoon et al., 1977), dimethyl sulfoxide (Odom and Maibach, 1976), and cinna-maldehyde (Kirton, 1978).

Introduction of allergens into the skin induces the immediate release of histamine from tissue-bound mast cells. Histamine activates venule endothelial cells by interacting with surface receptors. Activated endothelial cells initiate synthesis ofPAF and nitric oxide that induce relaxation of the vasculature. Red cells accumulate in the area creating a characteristic redness (erythema) in the skin. Concurrently, the endothelial cells retract creating openings in the venules. Plasma leaks from the vessel into the skin inducing edema or swelling in the skin (wheal). Bloodvessels at the periphery of the wheal often dilate creating a flare (Mathews, 1990).

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