Red reaction in triple response is due to: March 2011

Ans. B: Capillary dilation Red reaction or red line in triple reaction is due to dilatation of precapillary sphincters Triple response of Lewis It is a cutaneous response that occurs from firm stroking of the skin, which produces an initial red line, followed by a flare around that line, and then finally a wheal. The triple response of Lewis is due to the release of histamine. Histamine, or 2-(imidazol-4-yl) ethanamine, is a dibasic vasoactive amine that is located in most body tissues but is highly concentrated in the lungs, skin, and gastrointestinal tract. Histamine is derived from the decarboxylation of the aminoacid histidine, a reaction catalyzed by the enzyme Lhistidine decarboxylase. Histamine is a small molecule, stored as granules in mast cells and basophils. Mast cells and basophils are the effector cells involved in the immediate hypersensitivity response. Explanation :Injected intradermally histamine elicits the triple response consisting of: - Red spot: due to capillary dilation - Flare: redness in the surrounding area due to aeriolar dilatation mediated by axon reflex. - Wheal: due to exudation of fluid from capillaries and venules