Contact urticaria deviates from regular contact dermatitis in the type of clinical reaction, its time sequence, the causal agents, and the pathogenetic mechanism. True, the clinical reaction appears on the site of direct contact, usually the fingers, but consists of small, itching wheals, emerging within 10-20 minutes after contact and rapidly disappearing. Causal agents may be latex rubber in gloves, animal proteins or other food stuffs, the patients often being health care personnel, as well as chefs and workers in meat or fish industry.
Pertinent allergens are high-molecular, complete antigens, normally not absorbed through the skin. Therefore, preceding damage to the skin barrier. e.g. a discrete irritant dermatitis, would be a prerequisite. Pathogenetically, this is a type 1 allergy based on specific IgE antibodies, and the patients usually have an atopic constitution.
Testing for Contact urticaria
A regular 48 hour patch test will give a false negative reaction. Instead, a short term (20 min) prick test or scratch-chamber test with the suspected material, e.g. shrimps or rubber, will provoke a positive, immediate, wheal reaction. In latex cases, also specific IgE in blood may be demonstrated. It should be pointed out, however, that a non-immunological variant of contact urticaria also exists.