The skin lesions of urticarial disease are caused by an inflammatory reaction in the skin, causing leakage of capillaries in the dermis, and resulting in an edema which persists until the interstitial fluid is absorbed into the surrounding cells.
Urticaria are 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.
- Autoimmune urticaria: In the past decade, it has been noted that many cases of chronic idiopathic urticaria are the result of an autoimmune trigger. For example, roughly one third of patients with chronic urticaria spontaneously develop auto-antibodies directed at the receptor FcεRI located on skin mast cells.
Chronic stimulation of this receptor leads to chronic hives. Patients often have other autoimmune conditions such as autoimmune thyroiditis.
- Infections: Hive-like rashes commonly accompany viral illnesses, such as the common cold. They usually appear 3–5 days after the cold has started, and may even appear a few days after the cold has resolved.
- 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.
- Dietary histamine poisoning: This is termed scombroid food poisoning. Ingestion of free histamine released by bacterial decay in fish flesh may result in a rapid-onset allergic-type symptom complex which includes urticaria. However, the urticaria produced by scromboid is reported not to include wheals.
- Stress and chronic idiopathic urticaria: Chronic idiopathic urticaria has been anecdotally linked to stress since the 1940s. There is a large body of evidence demonstrating an association between this condition and both poor emotional well-being and reduced health related quality of life. More recent research has investigated hypotheses about stress as a causal factor in triggering the condition.
Evidence has been found for a link between stressful life events (e.g. bereavement, divorce etc...) and preliminary evidence has been reported for a link between posttraumatic stress and chronic idiopathic urticaria. Less is known about the individual experiences and characteristics of people who develop chronic idiopathic urticaria following stress.
Research into these factors in the relationship between stress and chronic idiopathic urticaria is ongoing by a number of researchers, including an online project currently being undertaken by researchers at the University of Plymouth.
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