Urticaria (or hives) is a skin condition, commonly caused by an allergic reaction, that is characterized by raised red skin wheals (welts). It is also known as uredo. Fact: date=September 2008 Often these diseased metamorphoses pop up in times of stress and tears. Often times this extreme occurrence arises on those with sensitive skin.

Wheals from urticaria can appear anywhere on the body, including the face, lips, tongue, throat, and ears. The wheals may vary in size from about 5 mm (0.2 inches) in diameter to the size of a dinner plate; they typically itch severely, sting, or burn, 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, but can also appear for other reasons, notably emotional stress. The rash can be triggered by quite innocent events, such as mere rubbing or exposure to cold.

Pathophysiology

Urticarial disease 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 the 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. Many drugs, for example morphine, can induce direct histamine release not involving any immunoglobulin molecule. Also, a diverse group of signaling substances called neuropeptides have been found to be involved in emotionally induced urticaria. Dominantly inherited cutaneous and neurocutaneous porphyrias (porphyria cutanea tarda, hereditary coproporphyria, variegate porphyria and erythropoietic protoporphyria) have been associated with solar urticaria. The occurrence of drug-induced solar urticaria may be associated with porphyrias. This may be caused by IgG binding not IgE.

Differential diagnosis

The rash that develops from poison ivy, poison oak, and poison sumac contact is commonly mistaken for urticaria. This rash is caused by contact with urushiol and results in a form of contact dermatitis called Urushiol-induced contact dermatitis. Urushiol is spread by contact, but can be washed off with a strong grease/oil dissolving detergent and cool water and rubbing ointments.