Nettle rash

Acute urticaria

Urticaria results from the release of histamine and other chemicals from under the skin's surface. This causes inflammation (swelling) of the tissues, and fluid to gather under the skin, which causes weals.

The trigger is unknown in around half the cases of acute urticaria. Recognised triggers include:

• an allergen, such as certain foods (e.g. nuts, shellfish or eggs), latex or a wasp sting
• infections, including colds or the flu
• irritants, such as chemicals, nettles or insect bites
• medicines, commonly non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, which are used to treat pain and fever
• physical triggers, such as pressure to the skin, change in temperature, sunlight, exercise or water

Chronic urticaria

Chronic urticaria is not caused by the same triggers as those for acute urticaria. It may start when your body's immune system attacks its own tissues (called autoimmunity). Antibodies (proteins that usually fight bacteria and viruses) attack the cells in your skin that contain histamine, causing it to be released. This produces urticaria. It is thought that about a third of chronic urticaria cases are autoimmune. It is not known why autoimmune urticaria develops.

In rarer cases, chronic urticaria can also be caused by other chronic illnesses and infections, such as thyroid disease, viral hepatitis or intestinal parasites.

Chronic urticaria often comes and goes. Many people find that certain factors can make it reappear. These include:

• stress
• alcohol
• caffeine
• warm temperature
• prolonged pressure on the skin (for example, through tight clothing)
• medications, especially NSAIDs and codeine