Triple response of Lewis

Overview

The Triple Response of Lewis is a physiological reaction of the skin to injury or irritation, first described by Sir Thomas Lewis in the early 20th century. This response is a classic demonstration of the body's inflammatory response and involves three distinct phases: redness, flare, and wheal.

Phases of the Triple Response

Redness

Illustration of the Triple Response of Lewis

The initial phase of the triple response is the appearance of a red line at the site of injury. This occurs within seconds of the stimulus and is due to the dilation of capillaries in the area. The redness is a direct result of the release of histamine from mast cells, which causes the blood vessels to widen, allowing more blood to flow into the area.

Flare

Following the initial redness, a flare or diffuse redness spreads out from the site of injury. This is caused by an axon reflex, where the stimulation of sensory nerves leads to the release of vasodilatory substances, such as substance P and calcitonin gene-related peptide (CGRP), from nerve endings. These substances further promote vasodilation and increase blood flow, resulting in the characteristic flare.

Wheal

The final phase is the formation of a wheal, which is a raised, swollen area at the site of injury. This occurs due to increased permeability of the blood vessels, allowing plasma to leak into the surrounding tissue. The wheal is also mediated by histamine and other inflammatory mediators, contributing to the swelling and edema observed.

Mechanism

The triple response is primarily mediated by the release of histamine and other inflammatory mediators from mast cells and sensory nerves. The process begins with the mechanical or chemical stimulation of the skin, which activates nociceptors and mast cells. The subsequent release of histamine causes vasodilation and increased vascular permeability, while the axon reflex further amplifies the response through the release of neuropeptides.

Clinical Significance

The Triple Response of Lewis is an important concept in understanding the body's inflammatory response and is used in clinical settings to assess the integrity of the nervous system and the function of mast cells. It is also a useful demonstration in medical education to illustrate the basic principles of inflammation and the role of the immune system in responding to injury.

Related Pages

• Inflammation
• Histamine
• Mast cell
• Nervous system
• Immune system