urticaria
Meaning of urticaria: A rash that behaves unlike most others: each welt lasts hours, fades without a trace, and reappears somewhere else.
Definition of urticaria
Urticaria is a skin reaction in which raised, itchy swellings appear suddenly, stay for a few hours, then fade leaving normal skin behind. The swellings may be small spots or broad pale-centered patches with a red rim, and new ones form as old ones settle. They arise when mast cells in the skin release histamine, which makes small vessels leak fluid into the tissue.
What is happening in the skin?
Mast cells sit throughout the skin loaded with granules of histamine and related chemicals. When something makes them empty those granules, the histamine widens nearby vessels and loosens their walls, so fluid escapes into the upper layer of skin. That trapped fluid lifts the surface into a wheal, presses the blood out of the center to leave it pale, and irritates nerve endings, producing the intense pruritus that gives the rash its character. Histamine is cleared quickly, so each swelling drains and vanishes within hours. When the same leak occurs in the deeper layer instead, the result is angioedema: puffy, poorly bordered swelling of the lips, eyelids, hands or throat lining that feels tight rather than itchy. The two often appear together, and the plain English name for the surface form is hives.
Is it always an allergy?
No, and assuming so sends many people down a long search for nothing. Sudden short-lived episodes can follow foods, medicines, insect stings or a viral illness, which is the commonest trigger in children. Cases running beyond six weeks are classed as chronic, and in most of those no outside trigger is ever identified; a share reflect the immune system acting against the mast cells themselves. Physical triggers form a group of their own, with cold, pressure, scratching, heat, sunlight, water and exercise each provoking wheals in susceptible people.
How is it told apart from lookalikes?
| Feature | Ordinary urticaria | Points elsewhere |
|---|---|---|
| Life of one spot | Hours, gone inside a day | Lasting beyond a day suggests urticarial vasculitis |
| What it leaves | Normal skin | Bruising or brown staining suggests another process |
| Main sensation | Itch | Burning or soreness suggests vasculitis |
| Depth | Upper skin | Deep tight swelling is angioedema |
Treatment centers on blocking the histamine signal, so a nonsedating antihistamine is the usual starting point, with other options where wheals persist. Wheals accompanied by swelling of the tongue or throat, wheeze, faintness or vomiting form part of anaphylaxis, a different and immediately dangerous reaction rather than a severe version of the rash.
Also known as
Used in a sentence
Urticaria appeared over the trunk within an hour of the infusion and settled overnight.
Good to know
Chronic urticaria is rarely caused by an allergy, and a single wheal that lasts beyond a day or leaves bruising behind points away from ordinary urticaria toward urticarial vasculitis.
Word origin
Where “urticaria” comes from
From the Latin urtica, a nettle, after the resemblance to a nettle sting.