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Contact urticaria is mediated by either allergic or nonallergic mechanisms. Allergic contact urticaria is mediated by IgE, and affected patients are usually atopic. Contact urticaria may be confused with allergic contact dermatitis when the erythematous wheals evolve into a vesicular dermatitis. However, the clinical history will reveal that the patient with contact urticaria developed itching at the time or soon after contact. [Rosenstock, p. 709] Contact urticaria is an effervescent swelling and erythema (a wheal) of the skin that peaks within minutes to hours after exposure and usually disappears within 24 hours. Contact urticaria is a localized reaction after skin contact with the causal substance. The reaction is either type I (allergic) or nonimmunologic (nonallergic). Generalized urticaria may follow the initial localized urticaria at the exposure site. [Asthma in the Workplace, p. 418-22] Some patients with contact urticaria have both immediate and delayed reactions. Other patients develop life-threatening anaphylactic reactions. IgE mediated contact urticaria is caused by the hair, urine, and saliva of animals and by latex, biocides, and some fruits and vegetables. Protein contact urticaria is an unusual type of contact urticaria that has been described in sandwich makers and other food workers who develop immediate itching, stinging, and vesiculation within minutes of handling meat or fish. [Marks, p. 310-11] Occupational contact urticaria accounts for about 30% of occupational allergic skin diseases in Finland. The most common agents causing immunologic contact urticaria (ICU) are food proteins, animal proteins, and natural rubber latex. Occupations most frequently affected are farmers, nurses, dental assistants, veterinarians, animal attendants, bakers, food handlers, and hairdressers. "Skin tests such as prick tests are positive, negative in controls, and determination of specific IGE is possible." Non-immunological contact urticaria (NICU) occurs without sensitization. Severe symptoms like anaphylaxis are not expected. Examples of chemicals causing NICU are benzoic acid and cinnamaldehyde. Protein contact dermatitis presents as eczema (chronic hand dermatitis) and is caused by type I hypersensitivity to protein or proteinaceous materials. [Kanerva, p. 217-26] Most reported cases of occupational contact urticaria are immunologic. Some low-molecular-weight chemicals rarely cause occupational contact urticaria: anhydrides, epoxy resins, polyfunctional aziridines, nickel, and reactive dyes. [Reference Link] "Differentiating between nonspecific irritant reactions and contact urticaria may be difficult." [Hayes, p. 1374]