Hives form when immune cells in your skin (called mast cells) release chemicals including histamine. This causes your blood vessels to open up and leak a little fluid under your skin. Histamine release can be triggered by lots of things and the cause isn't always clear.
- Infection from a virus is the most common cause of hives in children, especially if they last for more than 24 hours.
- Direct contact with plants or animals may cause hives in just one area of your body. This includes latex from rubber plants which is used in making rubber gloves and party balloons.
- Allergic reactions to food, medicines or insect stings can appear as hives. They usually occur within 1 to 2 hours of exposure and disappear in most cases within 6 to 8 hours.
- Hives can be caused by physical triggers, including cold (eg, cold air, water or ice), heat, sunlight (solar), vibration, rubbing or scratching your skin (dermatographism) and delayed pressure (eg, after carrying heavy bags).
- Rarely, exercise or sweating may cause hives.
- Stress rarely causes hives but may make your symptoms worse.
Identifying the cause of hives is tricky. An allergic reaction could be the cause if there are patterns to when they appear, eg:
- always within 2 hours of a meal
- always when exercising
- other symptoms occur around the same time (eg, stomach pain, vomiting, difficulty breathing or dizziness).
Ongoing hives lasting days at a time are almost never due to allergy, with the exception of some cases of allergy to medicines.