Pruritus or itching (Table 9) is a dermal reaction to diverse stimuli, including light touch, vibration, wool fibers, and a number of chemical mediators. Histamine released by mast cells is one of the most significant chemical mediators. Pruritus is a common symptom of primary skin diseases including allergic contact dermatitis. Less commonly it reflects a systemic reaction to drugs (e.g., NSAIDs, penicillin, and opioids) and other allergens.
Urticaria (Table 9) is a reaction to vasoactive substances (e.g., histamine) released by mast cells in the superficial dermis resulting in intradermal edema caused by capillary and venous vasodilation. The process could be an IgE-mediated type I hypersensitivity reaction; direct non-immune-mediated-activation of mast cells by drugs; drug-induced cyclooxygenase inhibition that activates mast cells by poorly understood mechanisms; or caused by stress and anxiety.
Angioedema (Table 9) is anaphylaxis of the subcutaneous tissues. It results from mast cell and basophil activation in the deeper dermis and subcutaneous tissues and is pathogenically related to urticaria which occurs at the epidermal-dermal junction. The causes of acute angioedema, which may be accompanied by pruritus and urticaria, include drugs and other allergens. Chronic angioedema is mostly idiopathic, rarely IgE mediated, and some cases are hereditary.
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