hives

Urticaria or hives is a relatively common form of allergic reaction that causes raised red skin welts. Urticaria is also known as nettle rash or uredo. These welts can be to 5 mm (0.2 inches) in diameter or more, itch severely, and often have a pale border. Urticaria is generally caused by direct contact with an allergenic substance, or an immune response to food or some other allergen. Hives can also be caused by stress.

Pathophysiology

The skin lesions of urticarial disease is caused by an inflammatory reaction in the skin, causing leakage from capillaries in the epidermis, resulting in an edema which persists until the interstitial fluid is absorbed into the surrounding cells.

Urticarial disease are thought to be caused by the release of histamine and other mediators of inflammation (cytokines) from cells in the skin. This process can be the result of an allergic or non-allergic reaction, differing in eliciting mechanism of histamine release.

• Allergic urticaria

Histamine and other pro-inflammatory substances are released from mast cells in the skin and tissues in response to the binding of allergen-bound IgE antibodies to high affinity cell surface receptors. Basophils and other inflammatory cells are also seen to release histamine and other mediators, and are thought to play an important role, especially in chronic urticarial diseases.

• Non-allergic urticaria

Mechanisms other than allergen-antibody interactions are known to cause histamine release from mast cells. For instance, a diverse group of signaling substances called neuropeptides have been found to be involved in emotionally induced urticaria.

Misconception

• Angioedema is a related condition where similar types of swelling occur in a lower layer of the skin, mouth or throat. It is not identical to urticaria. They may, however, occur together.
• The rash derived from poison-ivy is commonly mistaken for urticaria. Poison-ivy is caused by urushiol toxin. This resin can be spread by contact, but it is easily washed off.

Types

• Acute urticaria usually show up a few minutes after contact with the allergen and can last a few hours to several weeks. Food allergic reactions typically fit in this category. Common causes of reaction include consumption of shell fish, nuts, eggs, fish, acid derivitives, dye.
• Chronic urticaria refers to hives that persists for 6 weeks or more. There are no visual differences between acute and chronic urticaria. Some of the more severe chronic cases have lasted more than 20 years.
• Drug-induced urticaria has been known to result in severe cardiorespiratory failure. The anti-diabetic sulphonylurea glimepiride (trade name Amaryl®), in particular, has been documented to induce allergic reactions manifesting as urticaria. Other cases include aspirin, penicillin, Clotrimazole, sulfonamides and anticonvulsants.

• Physical urticarias is often categorized into the following.
  • Aquagenic: Reaction to water (rare)
  • Cholinergic: Reaction to body heat, such as when exercising or after a hot shower
  • Cold: Reaction to cold, such as ice, cold air or water
  • Delayed Pressure: Reaction to standing for long periods, bra-straps, belts
  • Dermatographism: Reaction when skin is scratched (very common)
  • Heat: Reaction to hot food or objects (rare)
  • Solar: Reaction to direct sunlight (rare)
  • Vibration: Reaction to vibration (rare)
  • Adrenergic: Reaction to adrenaline / noradrenaline (extremely rare)

External links

• Urticaria pictures and descriptions of conditions
• Urticaria conditions and descriptions