Urticaria occurs when histamine and other chemicals are released from under the skin's surface, causing the tissues to swell.

Short-term urticaria 

The triggers of short-term (acute) urticaria are unknown in around half of all cases.

Recognised triggers include:

• a food allergy to foods 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
• certain medications that can cause urticaria as a side effect, including antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs) and aspirin
• physical triggers, such as pressure to the skin, changes in temperature, sunlight, exercise or water

Long-term urticaria

Long-term (chronic) urticaria may occur when the 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, resulting in urticaria.

About a third to half of all chronic cases of urticaria are thought to be autoimmune related.

It's not known why autoimmune urticaria develops, although it can sometimes occur in combination with other autoimmune conditions, such as:

• rheumatoid arthritis – when the immune system attacks the joints
• lupus – when the immune system attacks the joints and skin, and people usually feel tired all the time

Chronic urticaria can also be linked to other chronic illnesses and infections, such as viral hepatitis (liver infection), intestinal parasites, an underactive thyroid gland (hypothyroidism) or an overactive thyroid gland (hyperthyroidism).

Chronic urticaria tends to come and go. Many people find that certain things make it reappear or make existing symptoms worse. Triggers include:

• stress
• alcohol 
• caffeine
• warm temperatures
• prolonged pressure on the skin – this can happen by wearing tight clothing
• medications – such as NSAIDs, and the painkiller codeine
• certain food additives – such as salicylates, which are found in tomatoes, orange juice and tea
• insect bites and stings
• exposure to heat, cold, pressure or water

ACE inhibitors that are often used to treat high blood pressure (hypertension) can be associated with deeper swellings of angioedema.