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Urticaria
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Chronic urticaria means itchy hives that come back in episodes. The cell behind them is the mast cell, an immune cell in the skin loaded with granules. When the episodes recur for less than six weeks, it is called acute spontaneous urticaria; when they last six weeks or more, chronic. It can occur on its own or along with angioedema.
What happens underneath is a release. Mast cells and basophils in the skin activate and degranulate, meaning they empty their granules, releasing histamine and other mediators. Those substances do two things to small blood vessels: they dilate them and make them more permeable. The vessel becomes porous and leaks fluid into the skin: that is what raises the hive. The fluid collects in the dermis, the middle layer of the skin, and forms the swelling you can see and feel.
One feature sets it apart from other rashes. Each hive is fleeting: it lasts less than 24 hours and clears without leaving a bruise or a mark.
In brief
Chronic urticaria is mast cell mediated and causes recurring episodes of hives, angioedema, or both; the acute spontaneous form lasts less than six weeks and the chronic form six weeks or more.
Mast cells and basophils in the skin degranulate, and histamine and other mediators dilate blood vessels and make them leaky, producing dermal edema and itchy hives.
The vessel becomes porous and leaks fluid into the skin: that is what raises the hive.
Each hive lasts less than 24 hours and leaves no bruise or discoloration.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingChronic Urticaria, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated