Urticaria is the medical term for a group of transient skin reactions commonly called hives, characterised principally by the sudden appearance of pruritic, raised wheals caused by localised swelling within the superficial dermis. A wheal is typically pale or erythematous, variable in size and shape, and often surrounded by normal or flushed skin. Individual lesions characteristically appear and disappear within 24 hours without leaving permanent marks, although new wheals may arise elsewhere as the condition continues.
Urticaria may occur alone or together with angioedema, which involves deeper dermal and subcutaneous or mucosal swelling and can affect the lips, eyelids, tongue, hands or feet. The term describes a clinical pattern rather than a single disease, because numerous immunological, physical, infectious, medication-related and other triggers can produce the same characteristic response.
The central mechanism of urticaria involves activation of cutaneous mast cells and release of mediators, particularly histamine, that increase vascular permeability and produce local oedema. Mast-cell activation can occur through immunoglobulin E-mediated allergic mechanisms, but many episodes are not caused by a conventional allergy. Other pathways include complement activation, neurogenic mechanisms and direct or indirect mast-cell activation by physical stimuli or medications.
Histamine acting primarily through H1 receptors contributes substantially to itching and vasodilation, which explains why H1-antihistamines are the main pharmacological treatment for many forms. The transient nature of ordinary wheals reflects the relatively rapid resolution of the vascular permeability changes, distinguishing urticaria from inflammatory dermatoses in which individual lesions commonly persist for days.
Urticaria is broadly classified according to duration and provoking mechanism. Acute urticaria lasts up to six weeks and may follow infections, foods, medications, insect exposures or other identifiable events, although a precise cause is frequently not established. Chronic urticaria persists or recurs for more than six weeks and is commonly divided into chronic spontaneous urticaria, in which wheals or angioedema occur without a consistent external physical trigger, and chronic inducible forms, in which reproducible stimuli such as cold, heat, pressure, exercise, vibration or water can provoke symptoms. Physical subtypes include dermatographism, where scratching or pressure produces linear wheals, and cholinergic urticaria, where small itchy wheals arise in association with increased body temperature and sweating.
Diagnosis is usually clinical and depends heavily on the appearance, behaviour and timing of lesions. A careful history considers onset, duration of individual wheals, associated angioedema, food or medication exposures, infections, physical triggers, occupational exposures and systemic symptoms. Extensive laboratory testing is generally not necessary for straightforward acute urticaria, and indiscriminate allergy testing can produce misleading results because many cases have no identifiable external allergen. Chronic or atypical disease may warrant targeted investigations based on the history and examination. Persistent lesions lasting longer than approximately 24 hours, especially when painful rather than itchy and followed by bruising or pigmentation, suggest alternative diagnoses such as urticarial vasculitis and may require further investigation, potentially including skin biopsy.
Management depends on severity, duration and cause. Avoidance of a clearly demonstrated trigger is useful, but broad dietary restriction or avoidance of numerous substances without evidence of causation can be unnecessary and burdensome. Second-generation, generally non-sedating H1-antihistamines are commonly used as first-line pharmacological therapy because they reduce histamine-mediated itching and whealing while causing less sedation than older antihistamines.
In chronic spontaneous urticaria that remains inadequately controlled, specialist treatment can include biologic therapy such as omalizumab, while other immunomodulatory treatments may be considered in selected refractory cases. Systemic corticosteroids may sometimes be used briefly for severe exacerbations under appropriate clinical supervision, but prolonged corticosteroid treatment is generally avoided because of its substantial adverse-effect burden.
Angioedema associated with urticaria should be distinguished from forms of angioedema caused by mechanisms other than histamine. Histamine-mediated angioedema often accompanies itchy wheals and generally responds to antihistamine-based treatment, whereas bradykinin-mediated angioedema, including forms associated with certain medications or hereditary conditions, typically lacks urticaria and responds to different therapies. Swelling of the tongue, throat or upper airway can compromise breathing and constitutes a medical emergency, particularly when accompanied by difficulty breathing, wheezing, faintness or rapidly progressive swelling. Anaphylaxis is also distinct from uncomplicated urticaria because it involves potentially life-threatening systemic manifestations such as airway compromise, respiratory difficulty, circulatory instability or significant gastrointestinal symptoms after an exposure.
Common misconceptions include the assumption that every episode of urticaria represents a food allergy or that identifying a single allergen is always possible. In reality, infections, autoimmune mechanisms, physical stimuli and spontaneous mast-cell activation account for many episodes, and chronic spontaneous urticaria is not simply a persistent form of food allergy. Stress can influence symptom severity in some individuals but should not automatically be regarded as the fundamental cause.
Scratching can aggravate symptoms through mechanical stimulation, particularly in dermatographism, but scratching does not mean that the eruption is necessarily caused by an external irritant. Urticaria is also different from eczema and contact dermatitis because its individual lesions are typically transient rather than remaining fixed in the same location for several days.
The clinical significance of urticaria ranges from a short-lived, self-limiting eruption to a chronic disorder that substantially affects sleep, concentration, work, school and quality of life. Acute episodes frequently resolve spontaneously, whereas chronic urticaria can fluctuate over months or years and may have no identifiable initiating event. Chronic disease can create considerable psychological and practical burden even when it is not medically dangerous. Prognosis depends on the underlying subtype and cause rather than on the appearance of the rash alone. Accurate recognition is important because it prevents unnecessary avoidance diets and investigations, facilitates appropriate antihistamine treatment, identifies patients who require specialist management and ensures that potentially dangerous airway swelling or systemic allergic reactions are not mistaken for uncomplicated hives.
