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from Guyton and Hall textbook of medical physiology, 4th South Asia edition (Gastrointestinal system) Writer five mark short note on triple response.

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Guyton Hall Medical Physiology triple response Lewis dermatographism

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Triple Response of Lewis

The triple response is a characteristic local vascular reaction produced when the skin is stroked firmly with a blunt instrument or when histamine is injected intradermally. It was described by Sir Thomas Lewis.

Components

  1. Red line
    • Appears within a few seconds at the site of stroking.
    • Caused by local dilatation of capillaries due to direct mechanical injury and release of local vasodilator substances, especially histamine.
  2. Flare
    • A diffuse red area appears around the red line.
    • It results from arteriolar dilatation in the surrounding skin.
    • This is mediated by an axon reflex: stimulation of sensory nerve endings causes antidromic impulses in collateral branches, releasing vasodilator substances.
  3. Wheal
    • A localized, raised, pale swelling develops along the line within a few minutes.
    • It is due to increased capillary permeability, causing leakage of plasma and proteins into the interstitial space, producing local edema.

Mechanism and significance

  • Mechanical injury causes local release of histamine from mast cells.
  • Histamine causes vasodilatation and increases capillary permeability.
  • The flare requires intact sensory nerve endings; therefore, it is absent in denervated skin.
  • The response demonstrates the role of local chemical mediators and axon reflexes in regulating cutaneous circulation.
Sequence:
Red line → Flare → Wheal
Note: Although you mentioned the gastrointestinal system, triple response is a topic of local skin circulation and histamine action, usually covered under cardiovascular/local blood flow physiology.
Source: Guyton and Hall Textbook of Medical Physiology, section “Vasodilators,” p. 220.
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