JOHN RADCLIFFE HOSPITAL / SCIENCE PHOTO LIBRARY JOHN RADCLIFFE HOSPITAL / SCIENCE PHOTO LIBRARY
Allergy testing: red wheals resulting from a positive patch test on a patient's back. A patch test is used to discover which of a number of possible substances is responsible for a patient's allergy. Small quantities of different allergens are applied either to light scratches on the skin of the arm or back, or to undamaged skin which is then covered with adhesive tape. In common allergies the offending substance causes a patch of swelling and a typical red flare in 5 to 15 minutes. A delayed reaction may take 24 to 72 hours to develop.
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