Wheal skin
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Understanding Wheal Formation in Skin: Mechanisms and Measurement
Introduction to Wheal Formation in Skin
Wheals, also known as hives, are raised, itchy areas of skin that are often a result of allergic reactions. They are a common feature in conditions such as urticaria and can be induced by various stimuli, including allergens, physical factors, and internal immunologic responses.
Mechanisms of Wheal Formation
Immunologic Pathways
Wheal formation can be triggered through immunologic pathways. One method involves the injection of serums from hypersensitive patients into the skin of a sensitized subject. When the subject consumes the specific allergen, a wheal forms at the sensitized site, demonstrating the absorption of undigested proteins from the gastrointestinal tract . This method highlights the role of immune responses in wheal formation.
Role of Mast Cells
Mast cells play a crucial role in the development of wheals. These cells, particularly the MCTC subtype, are located in the upper dermis, making them ideally situated for wheal formation and sensory nerve stimulation. Mast cell degranulation releases histamine and other mediators, leading to the characteristic wheal and flare reactions . Increased mast cell activity and numbers are observed in both lesional and non-lesional skin in chronic spontaneous urticaria (CSU) and inducible urticaria .
Substance P and Other Peptides
Substance P and related peptides can induce wheal and flare reactions when injected into the skin. The presence of basic residues in these peptides is essential for their activity. Substance P is significantly more potent than histamine in producing these reactions, indicating its strong role in wheal formation .
Measurement of Wheal Responses
Skin Prick Tests (SPT)
The evaluation of wheal responses in skin prick tests (SPT) is crucial for diagnosing allergies. The longest diameter of the wheal is considered the optimal measurement parameter, as it correlates more significantly with the wheal surface area compared to the mean diameter. This method is not only more accurate but also easier and faster to perform .
Electrical Impedance Technique
An innovative method for assessing wheals involves measuring the electrical impedance of the skin. This technique can objectively characterize and quantify wheal reactions, distinguishing them from other types of skin reactions such as allergic and irritant contact reactions .
Wheal Formation in Newborns and Infants
The capacity of newborn and young infant skin to form wheals is a subject of debate. Studies indicate that while full-term newborns react to histamine with erythema, they do not typically form wheals. However, passively sensitized skin sites in newborns can show wheal formation upon exposure to specific allergens, suggesting that their skin can demonstrate antigen-antibody reactions 35.
Correlation with Itch and Flare
The relationship between wheal formation, itch, and flare reactions is complex. Histamine-induced wheals and flares are often accompanied by itch, which varies with the method of histamine application. Skin prick tests tend to produce more prolonged itch sensations compared to iontophoresis, which induces transient itch. This difference is attributed to the duration and depth of histamine exposure in the skin .
Conclusion
Wheal formation in the skin is a multifaceted process involving immunologic pathways, mast cell activity, and various peptides. Accurate measurement techniques such as the longest diameter in SPTs and electrical impedance provide valuable tools for assessing wheal responses. Understanding these mechanisms and measurement methods is essential for diagnosing and managing allergic reactions and other conditions associated with wheal formation.
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