urticaria - Pathophysiology

An allergic reaction affecting the skin, resulting in the formation of red, itching weals and blisters; also known as nettle rash or hives. It can be provided by direct contact with an irritant, or occur as part of a generalized response to an allergen.

ICD-9	708
DiseasesDB	13606
eMedicine	emerg/628

Urticaria or hives is a relatively common form of allergic reaction that causes raised red skin welts. Urticaria is generally caused by direct contact with an allergenic substance, or an immune response to food or some other allergen.

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. Common causes of reaction include consumption of shell fish, nuts, eggs, fish, acid derivitives, dye. 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)