Colour Change In the Human Histamine Wheal; A Sign of Desensitized Histamine Vasoconstrictory Receptors

Background: The aim was to find the cause and consequences of a colour change in histamine wheals found after ordinary histamine skin prick tests (SPTs) (10 mg/ml). A rapid change to a darker red colour from the 18th and to the 20th minute has been demonstrated by using a digital image-processing technique called LYYN and ImageJ to yield numerical values (1).

Methods: Repeated histamine SPTs in the middle of the site for earlier performed histamine SPTs in humans. Calculations of the sizes of photographed wheals. Histamine solutions perfused in isolated rabbit ears. 

Results: Histamine SPT performed 90 min or 6 hours apart from initial histamine SPTs evoked a ring of wheal peripherally around the site of the initial wheal or no wheal at all. The initial wheals had at those times disappeared. Histamine perfusion in isolated rabbit ears indicated first vasoconstriction and after a mean of 17 min vasodilatation in post-capillary vessels despite continued histamine perfusion. 

Conclusions: The results indicate that total desensitization of histamine-1 receptors in the wheal is the cause of the colour change in human histamine SPTs and that such desensitization lasts long time. If histamine released at allergen provocations also evokes such a long-lasting desensitization and post-capillary vasodilatation it opens new aspects on vascular events in allergic reactions.

Reference: 1. Holm O, Lindell E, Malm L. Colour change in the human histamine wheal made visible by LYYN: a technique to enhance colour differences. Skin Res Technol. 2010 Nov;16(4):385-9.