Urticaria or hives is commonly caused by the release of histamine and other chemical messengers in the skin surface. The release of these chemicals leads to the symptoms of allergy such as redness, itchiness and swelling.

Causes

Some common causes of hives include allergies to:

• Food items such as peanuts, shellfish, eggs, milk and cheese
• Dust mites
• Pollen
• Chemical irritants
• Nickel and plating on costume jewellery
• Latex
• Hot or cold temperature
• Insect bites or stings
• Emotional changes and stress
• Pressure caused by elastic bands, tight clothing or straps.
• Scratched skin surface
• Medications such as the glimepiride prescribed for diabetes, pain relievers such as non-steroidal anti-inflammatory drugs (NSAIDs) and aspirin, and antibacterials such as penicillin and sulphonamide
• Long term urticaria may occur with autoimmune diseases such as lupus or rheumatoid arthritis
• Caffeine
• Alcohol

Pathology of hives

The skin lesions caused by hives are usually raised, red and itchy bumps. The reaction begins when cutaneous mast cells and basophils release histamine and other inflammatory mediators at the skin's surface. The mast cells are stimulated to release these chemicals when Immunoglobulin E (IgE) antibodies bind to an allergen and effectively flag it up as a foreign body.

Histamine triggers the dilation of the blood vessels across the skin leading to its warm and reddish appearance. In addition, the dilated blood vessels can become leaky and release fluids that cause swelling or edema. This often remains until the excess fluid is absorbed back by cells surrounding the swelling.