Which allergy test involves placing a patch soaked with potential antigens on the skin and observing immune response after several days?

• A. Scratch test
• B. Prick test
• C. Skin patch test
• D. Intradermal allergy testing

Answer: (C)

This tests for delayed-type, cell-mediated allergic reactions. A patch containing potential allergens is placed on the skin (usually the back) and left in place for about 48 hours. After removal, the skin is checked at 48 to 72 hours (sometimes up to 96 hours) for an eczematous reaction under the patches. This type of response is driven by T cells and represents a type IV hypersensitivity, which develops over days. By contrast, scratch, prick, and intradermal tests are aimed at immediate, IgE-mediated reactions and are read within minutes to a few hours after introducing the allergen. So, applying a patch and observing the reaction after several days is characteristic of the skin patch test.