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MODEL RELEASED. Allergy test. Arm of a patient undergoing a skin prick allergy test. This test is used to identify substances (allergens) that cause an allergic reaction in the patient. Small amounts of suspected allergy-causing substances are placed on the skin, then the skin is pricked with the tip of a scalpel blade (as seen here) in order to introduce them under its surface. The skin is then observed for signs of a reaction, which usually develop within several minutes. These include redness and swelling. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR P. MARAZZI)