Patch test is what type of hypersensitivity reaction?
**Core Concept**
A patch test is a diagnostic tool used to identify delayed-type hypersensitivity reactions, specifically contact dermatitis caused by allergens such as metals, dyes, or other substances that come into contact with the skin. This type of reaction is mediated by T lymphocytes and involves a cell-mediated immune response.
**Why the Correct Answer is Right**
The patch test involves applying small amounts of potential allergens to the skin and observing the reaction after 48-72 hours. The immune system recognizes the allergen as foreign and mounts a delayed response, resulting in inflammation and skin lesions. This reaction is characterized by the activation of T helper 1 (Th1) cells and the release of cytokines such as interferon-gamma (IFN-γ), which contribute to the inflammatory response.
**Why Each Wrong Option is Incorrect**
**Option A:** Type I hypersensitivity reaction - This is incorrect because type I reactions are immediate hypersensitivity reactions, mediated by IgE antibodies and mast cells, and typically involve symptoms such as anaphylaxis or urticaria.
**Option B:** Type IV hypersensitivity reaction - This is incorrect because while type IV reactions are also cell-mediated, they typically involve the activation of T cells and the release of cytokines, but are not limited to skin reactions.
**Option C:** Type III hypersensitivity reaction - This is incorrect because type III reactions involve the formation of immune complexes and activation of complement, leading to inflammation and tissue damage, but are not typically associated with skin reactions.
**Clinical Pearl / High-Yield Fact**
Remember that patch tests are used to diagnose contact dermatitis caused by allergens such as metals, dyes, or other substances that come into contact with the skin. This is a common and important consideration in the diagnosis and management of skin disorders.
**Correct Answer: D. Delayed-type hypersensitivity reaction.