Urticaria, commonly called hives, is a rash of raised, itchy weals caused by the release of histamine and other substances into the dermis. Its defining feature is movement: individual weals typically come up and fade within hours, appearing somewhere else while the previous ones settle.
What is actually happening in the skin?
Mast cells sit throughout the dermis. They are immune cells that hold granules of histamine and other active substances, and when they are triggered they release that contents into the surrounding tissue.
The consequences are quick. Small blood vessels dilate, which produces redness. They also become leaky, so fluid moves out into the dermis, and that fluid lifts the skin into the raised area clinicians call a weal. Nerve endings register the histamine as itch, which in urticaria is often described as burning or stinging rather than as the scratch-provoking itch of eczema.
Then it reverses. The fluid drains away, the vessels recover, and the weal fades leaving skin that looks entirely normal. That whole cycle typically takes hours, which is why urticaria is the rash people photograph in a waiting room and cannot show by the time they are seen.
What triggers the mast cells in urticaria varies enormously. Infections are a frequent precipitant, particularly in shorter-lived episodes. Certain medicines can do it. Physical stimuli including pressure, cold, heat and sunlight are recognised triggers in some people. Allergy accounts for a share of urticaria. And in a substantial proportion, especially those lasting a long time, no external cause is found at all, which is a recognised pattern rather than an incomplete answer.
How would a dermatologist describe it?
A weal, sometimes spelled wheal, is the raised area itself and the basic unit of urticaria. Clinicians describe weals by size, shape and, above all, by how long an individual one lasts. That last point is asked about with unusual precision, because a weal that outstays the usual short span suggests something other than ordinary urticaria and changes the investigation.
Angioedema describes swelling in the deeper dermis and the tissue below it, typically affecting lips, eyelids and hands. It accompanies urticaria in some people and feels different: more swelling and discomfort, less itch.
Acute and chronic are used with a specific threshold, a conventional number of weeks that a clinician can state precisely, and the two categories are approached differently. Chronic urticaria is subdivided further into spontaneous and inducible forms, the latter meaning that a physical trigger reproducibly brings it on.
Dermographism is worth knowing: the term describes weals raised by stroking the skin, so that firm pressure with a blunt point leaves a visible line. A clinician may demonstrate it during an examination, and seeing it happen is oddly reassuring, because it makes the mechanism concrete.
What usually happens at a consultation?
Timing dominates the history. How long has this been going on overall, how long does a single spot last, does the skin return to normal in between, is there any swelling of lips or eyelids, what was happening in the days before it began. Questions about recent infections, new medicines and physical triggers are routine.
The examination may find nothing at all if the skin is currently clear, which is entirely normal and is not a wasted appointment. This is where photographs earn their place. A clinician may test for dermographism, and will look for signs of angioedema.
Investigation depends on the picture. Shorter episodes with an evident precipitant often need very little; longer-running urticaria is approached differently. A clinician may explain that the aim is to characterise the pattern rather than to find a single culprit, which is frequently the honest position.
What is worth asking?
Ask how long an individual weal should be lasting, and what it would mean if yours last longer. This is the single most informative observation you can make at home, and knowing what the clinician is looking for turns you into a useful observer.
Ask whether what you have is being classed as acute or chronic, and what that implies for what happens next. The two paths diverge, and knowing which one you are on explains a great deal about why some investigations are being done and others are not.
Enquire about swelling specifically, including what would count as urgent. Angioedema involving the mouth or throat is handled differently from a rash on the arms, and having that boundary stated plainly in the room is far better than trying to judge it at two in the morning.
Where no cause is found, ask whether that is expected in a case like yours. Hearing that a negative result is a normal outcome in urticaria, rather than a failure to look properly, saves people from pursuing investigations that were never going to help.
What this page does not do
It does not tell you what is triggering your skin, and in many cases no page or test will. Nothing here recommends a treatment, an antihistamine, an avoidance strategy or a dose, all of which depend on the individual and on a clinical assessment.
Common questions
How is urticaria different from other itchy rashes?
Movement is the distinguishing feature. Individual weals in urticaria typically rise and fade within hours, leaving normal skin behind, while the rash as a whole continues elsewhere. Most other rashes stay put and change slowly, which is why clinicians ask how long a single spot lasts.
Does urticaria always mean an allergy?
No, and this is a common assumption worth setting aside. Allergy is one recognised trigger among several, and in a great many cases, particularly longer-running ones, no specific external cause is ever identified. That absence is a well-described outcome rather than a failed investigation.
What is the difference between acute and chronic urticaria?
The distinction is made on duration, using a conventional threshold measured in weeks. Acute urticaria settles within that period; chronic urticaria continues beyond it. The two are investigated and managed quite differently, which is why the timeline gets asked about so precisely. Your clinician can tell you where the line falls.
What is angioedema?
It is swelling in the deeper tissue rather than in the upper dermis, often affecting the lips, eyelids or hands, and it can occur alongside urticaria. It feels more like swelling than itching. Swelling involving the mouth, tongue or throat needs urgent assessment rather than an outpatient appointment.
Why do photographs matter so much for hives?
Because the rash has usually gone by the time an appointment arrives. A dated photograph taken while weals were visible may be the only direct evidence a clinician gets to see, and it is often more useful than any description you can give from memory.
Sources
- Hives — conditions — NHS (United Kingdom) · accessed
- Patient information on skin conditions — British Association of Dermatologists (United Kingdom) · accessed
Last updated . How this page was written.