Triple response

Triple Response of Lewis is a physiological phenomenon observed when the skin is subjected to a firm stroke, such as with a blunt object. This response was first described by Sir Thomas Lewis in 1927 and is a classic demonstration of the complex interactions between the nervous system and inflammatory processes in the skin. The Triple Response consists of three distinct components: red line, flare, and wheal, each representing different mechanisms of action within the skin's response to minor injury.

Components

The Triple Response of Lewis is characterized by three primary reactions:

1. Red Line: The initial reaction is the appearance of a red line along the site of the stroke, which is due to the direct damage to the capillaries and the immediate release of histamine. This causes the capillaries to dilate, leading to redness.
2. Flare: Following the red line, a reddish flare appears surrounding the area. This is not directly caused by the physical stroke but is a neurogenic response. It involves the reflex dilation of arterioles, extending beyond the immediate area of the stroke, due to the axon reflex mediated by sensory nerves.
3. Wheal: The final component is the formation of a wheal, a localized area of edema (swelling). The wheal is the result of increased vascular permeability, allowing fluid to accumulate in the extracellular space.

Mechanism

The Triple Response of Lewis illustrates the body's immediate reaction to minor trauma and involves a complex interplay of neurological and immunological processes. The red line is primarily a direct histamine-mediated response. The flare, however, involves the activation of sensory nerves, which release neuropeptides such as Substance P and CGRP (Calcitonin Gene-Related Peptide), leading to vasodilation at a distance from the injury. The wheal results from the action of histamine and other mediators on the vascular endothelium, increasing permeability and allowing fluid leakage into the surrounding tissue.

Clinical Significance

The Triple Response of Lewis can be used to assess the integrity of the peripheral nervous system and the inflammatory response of the skin. It has implications in diagnosing conditions that affect sensory nerves or alter the inflammatory processes. Additionally, understanding this response is crucial for research into pain, inflammation, and the skin's reaction to injury.

See Also

• Histamine
• Inflammation
• Edema
• Neurogenic inflammation

‎