Allergic urticaria is the least common form, although it is somewhat more common in children than in adults. It is caused by the immune systems overreaction to foods, drugs, infection, insect stings, blood transfusions or other substances. Foods such as eggs, nuts and shellfish, and drugs such as penicillin and sulfa are common causes of allergic or immunologic urticaria. Recent studies also suggest that some cases of chronic urticaria are caused by autoimmune mechanisms, when the patient develops immune reactions to components of his or her skin. Non-allergic urticaria are those types of urticaria where a clear-cut allergic basis cannot be proven. These take many forms like, dermographism is urticaria that develops when the skin is stroked with a firm object. Cold-induced urticaria appears after a person is exposed to low temperatures for example, after a plunge into a swimming pool or when an ice cube is placed against the skin. Cholinergic urticaria, which is associated with exerc