Intradermal injection of histamine into the forearm skin produces a central red spot, a surrounding irregular flare, and a raised wheal. The flare component of this triple response is mediated by:
- A An axon reflex through sensory nerve endings releasing substance P and CGRP ✓
- B Direct histamine action on vascular smooth muscle causing vasoconstriction
- C Histamine acting on H2 receptors of dermal lymphatics
- D Local release of nitric oxide from endothelial cells
Correct answer: A. An axon reflex through sensory nerve endings releasing substance P and CGRP
Explanation
The Lewis triple response consists of a red spot (capillary dilatation), the flare (a spreading erythema caused by an axon reflex in sensory C-fibres releasing neuropeptides such as substance P and CGRP), and the wheal (oedema from increased venular permeability). The flare persists even after local histamine action is confined, because it is neurogenic, and it is abolished by denervation or local anaesthetic, not by H1 blockade alone.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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