Urticaria is caused by the release of histamine and other chemicals from under the skin's surface, causing the tissues to swell.
Acute urticaria
What triggers acute urticaria is unknown in around half of cases. Recognised triggers include:
- an allergic reaction to food, such as peanuts, shellfish, eggs and cheese
- an allergic reaction to environmental factors such as pollen, dust mites or chemicals
- an allergic reaction to latex, which can be a common problem in healthcare workers
- infections, which can range from relatively trivial such as a cold to very serious such as HIV
- insect bites and stings
- emotional stress
- some medications, which can cause urticaria as a side effect, including antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), aspirin, and antihistamines
- physical triggers, such as pressure to the skin, change in temperature, sunlight, exercise or water
Long-term urticaria
Chronic urticaria may start when your body's immune system attacks its own tissues. This is known as an autoimmune reaction. Antibodies (proteins that usually fight bacteria and viruses) then trigger the release of histamine.
This produces urticaria. It is thought that about a third to half of chronic urticaria cases are autoimmune.
It is not known why autoimmune urticaria develops, although it can often develop in combination with other autoimmune conditions such as:
- rheumatoid arthritis – the immune system attacks the joints
- lupus – the immune system attacks the joints and skin, and people usually feel very tired all the time
In rarer cases, chronic urticaria can also be caused by other chronic illnesses and infections, such as an underactive thyroid gland or overactive thyroid gland, viral hepatitis (liver infection) or intestinal parasites.
Chronic urticaria often comes and goes. Many people find that certain things make it reappear or make existing symptoms worse. These triggers include:
- stress
- alcohol
- caffeine
- warm temperatures
- prolonged pressure on the skin (for example, from tight clothing)
- medications such as NSAIDs, the painkiller codeine and ACE inhibitors (used to treat high blood pressure)
- certain food additives such as salicylates, which are found in tomatoes, orange juice and tea
- insect bites and stings
- stress
- exposure to heat, cold, pressure or water
