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Larry Chiaramonte, Allergy & Immunology, answeredAllergic urticaria is less common than nonallergic, although it is somewhat more common in children than in adults. It is caused by the immune system's overreaction to foods, drugs, infections, and various substances. Foods such as eggs, nuts, and shellfish, and medications such as penicillin and sulfa are common causes of allergic immunologic urticaria. Recent studies also suggest that some cases of chronic urticaria are caused by autoimmune mechanisms, when patients develop immune reactions to components of their own skin.