The Lewis triple response illustrates the effects of hista-mine on vascular smooth muscle, vascular endothelium, and sensory nerve endings. Intradermal injection of as little as !0 ¡g histamine produces three distinct effects:
1. Dilation of capillaries in the immediate vicinity of the injection results in a local red or blue region (flush).
2. Dilation of arterioles results in an irregular red flare over an area that is generally wider than that due to the capillary dilation. The flare probably results from an axon reflex in which histamine stimulates autonomic nerve endings, causing release of vasodilatory mediators.
3. Swelling (wheal) appears in the area of capillary dilation. The increased permeability of the blood vessels in this region is responsible for the edema.
In addition to the flush, wheal, and flare, transient pain and itching result from the effects of histamine on sensory nerve endings. In sensitized individuals, intradermal injection of specific antigens produces a wheal; this reaction is the basis for a skin test to quantify the extent of the allergic response.
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