The fundamental application of skin testing is to detect mast cell-bound immunoglobulin (IgE) speci c for the allergen introduced in the skin (i.e., detection of sensitization). The allergy skin test in this context can be viewed as a human bioassay to detect cell-bound allergen-speci c IgE analogous to in vitro tests that detect circulating allergen-speci c IgE. In contrast to in vitro tests, cutaneous allergen-induced in ammation also re ects allergic disease in miniature. The immediate cutaneous in ammatory response occurs due to the release of mast cell in ammatory mediators triggered by binding of the allergen to mast cell-bound allergen-speci c IgE, thus mimicking the natural disease. Skin testing also can help to determine the clinical relevance of an allergen. It can indicate the likelihood that other organs, or the organism as a whole, will exhibit an allergic in ammatory response to the same or a cross-reacting allergen after a systemic, local, natural, or iatrogenic exposure.