An intradermal injection of histamine into the forearm of a healthy volunteer produces a sequence of local changes. The correct order of events in the Lewis triple response is:

• A Wheal, then red spot, then flare
• B Red spot, then wheal, then flare
• C Flare, then wheal, then red spot
• D Red spot, then flare, then wheal ✓

Explanation

The triple response begins with a red spot within seconds, caused by direct capillary dilatation from injected histamine. Next comes the flare, a bright irregular reddening spreading several centimetres beyond the injection site, produced by axon reflex mediated vasodilatation of arterioles. Finally a wheal appears at the injection site within minutes, due to histamine acting on H1 receptors to increase postcapillary venule permeability with local oedema.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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