INFLAMMATORY AND IMMUNE-MEDIATED SKIN DISORDERS

Urticaria

Etiology and risk factors

• Acute triggers include viral infection, foods, medications, insect stings, and contact exposures. Chronic spontaneous urticaria often has no external trigger and may involve functional autoantibodies or autoimmune mast-cell activation (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Inducible forms follow pressure, cold, heat, exercise/cholinergic stimulation, vibration, water, sunlight, or scratching. Acute is less than 6 weeks; chronic is 6 weeks or longer (Gohara et al., 2012; Pincus & McCalmont, 2019).

Pathophysiology

• Mast-cell degranulation releases histamine and other mediators, causing superficial dermal vasodilation and increased permeability. Local edema forms a wheal with surrounding erythema (Norris, 2020; Pincus & McCalmont, 2019).
• Deeper dermal/subcutaneous mediator effects cause angioedema. Bradykinin-mediated angioedema is a different pathway and usually occurs without hives or pruritus (Norris, 2020; Pincus & McCalmont, 2019).

Clinical manifestations

• Abrupt raised, blanching, intensely pruritic wheals of variable size and shape that move or disappear within hours. No primary scale, crust, vesicle, or pigment remains (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Angioedema causes deeper, often painful swelling of lips, eyelids, tongue, hands, feet, genitalia, or airway. Anaphylaxis adds respiratory compromise, hypotension, or significant multisystem symptoms (Gohara et al., 2012; Pincus & McCalmont, 2019).

Findings that argue against or redirect

• A fixed lesion lasting more than 24 hours, pain/burning, purpura, fever, joint/renal symptoms, or residual pigment suggests urticarial vasculitis or another inflammatory disease (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Fixed target lesions favor EM; contact-distribution vesicles favor dermatitis; nonpruritic angioedema without wheals suggests bradykinin-mediated disease (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Normal testing does not exclude chronic spontaneous urticaria, which is a clinical diagnosis (Gohara et al., 2012; Pincus & McCalmont, 2019).

Diagnostic evaluation

• Confirm the duration of each lesion, total disease duration, triggers, medications including NSAIDs and ACE inhibitors, physical stimuli, infection symptoms, and anaphylaxis features. Photographs help when lesions have resolved (Pincus & McCalmont, 2019; Zuberbier et al., 2026).
• Do not order routine testing for uncomplicated acute spontaneous urticaria unless the history suggests a specific allergy or drug reaction. For chronic spontaneous urticaria, use a limited evaluation such as CBC with differential and CRP or ESR, then add tests only when the history and examination indicate a specific cause or comorbidity (Pincus & McCalmont, 2019; Zuberbier et al., 2026).
• Consider complement and C1-inhibitor testing for recurrent angioedema without wheals and biopsy lesions that persist longer than 24 hours, are painful or purpuric, or leave residual pigment when urticarial vasculitis is suspected (Pincus & McCalmont, 2019; Zuberbier et al., 2026).

Expected diagnostic and laboratory findings

• Clinical finding: transient blanching wheal with no epidermal surface change. Biopsy, rarely needed, shows dermal edema and sparse perivascular inflammation without primary epidermal injury (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Elevated tryptase can support recent systemic mast-cell activation when collected at the correct time, but a normal result does not exclude anaphylaxis (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Low complement or vasculitic histology redirects toward urticarial vasculitis (Gohara et al., 2012; Pincus & McCalmont, 2019).

Differential diagnosis

• Urticarial vasculitis: lesions persist over 24 hours, hurt/burn, may be purpuric, and leave residual pigment; biopsy shows vasculitis (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Erythema multiforme: fixed target lesions lasting days (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Allergic contact dermatitis: delayed exposure-linked eczematous/vesicular surface change (Gohara et al., 2012; Pincus & McCalmont, 2019).
• Hereditary/ACE-inhibitor angioedema: deeper nonpruritic swelling without wheals; complement/medication history helps distinguish (Gohara et al., 2012; Pincus & McCalmont, 2019).

Treatment and management

• Avoid an identified trigger and begin a second-generation H1 antihistamine. If control is inadequate, increase the dose of the same agent up to fourfold under clinician direction rather than combining multiple H1 antihistamines (Pincus & McCalmont, 2019; Zuberbier et al., 2026).
• Add omalizumab for chronic spontaneous urticaria that remains uncontrolled on high-dose second-generation H1 antihistamine. Cyclosporine may be considered by a specialist for severe disease refractory to antihistamines and omalizumab (Pincus & McCalmont, 2019; Zuberbier et al., 2026).
• Avoid long-term systemic corticosteroids. A brief rescue course, generally no longer than 10 days, may be considered for a severe acute exacerbation (Pincus & McCalmont, 2019; Zuberbier et al., 2026).
• Epinephrine is first-line for anaphylaxis or threatened airway or circulation; antihistamines do not replace epinephrine. Stop an offending medication when clinically safe and appropriate (Pincus & McCalmont, 2019; Zuberbier et al., 2026).

Additional complications and red flags

• Angioedema without hives requires evaluation for bradykinin-mediated causes because antihistamines/epinephrine may not address the underlying pathway, although airway management remains urgent (Pincus & McCalmont, 2019; Zuberbier et al., 2026).

Clinical Reasoning Priorities

• Recognize first: Before treating the skin findings, assess for anaphylaxis. Hypotension, respiratory distress, bronchospasm, stridor, throat tightness, tongue or laryngeal swelling, syncope, or rapidly progressive multisystem symptoms require emergency treatment (Zuberbier et al., 2026).
• Reassess probability: Transient, pruritic, migratory wheals that resolve without residual change, usually within 24 hours at an individual site, support urticaria. Fixed lesions lasting longer than 24 hours, pain rather than itch, bruising, purpura, fever, or systemic inflammation should redirect the differential (Gohara et al., 2012; Zuberbier et al., 2026).
• Do not miss: Anaphylaxis and airway-threatening angioedema are the immediate hazards. Urticarial vasculitis, mast-cell disorders, autoinflammatory disease, and medication reactions should be considered when morphology or systemic findings are atypical (Zuberbier et al., 2026).
• Use decisive testing: Diagnose typical acute urticaria clinically and avoid broad routine testing without a directed indication. Persistent or atypical disease warrants history-driven evaluation for inducible triggers, inflammatory disease, vasculitis, or another competing diagnosis (Zuberbier et al., 2026).
• Act on time-sensitive decisions: Administer intramuscular epinephrine immediately for anaphylaxis; antihistamines do not replace epinephrine. Treat isolated urticaria primarily with a second-generation H1 antihistamine and address identifiable triggers when feasible (Zuberbier et al., 2026).

Reinforce the inflammatory skin differential in the Infection, Immunodeficiency, and Inflammatory Skin Disorders Quiz.

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References

Gohara, M. A., Schaffer, J. V., Abbasi, N. R., Kingsley, M. M., Sheehan, J. M., & Arndt, K. A. (2012). Inflammatory dermatoses (rashes). In M. C. Henderson, L. M. Tierney, Jr., & G. W. Smetana (Eds.), The patient history: An evidence-based approach to differential diagnosis (2nd ed., pp. 239–255). McGraw-Hill Medical.

Norris, T. L. (2020). Porth’s essentials of pathophysiology (5th ed.). Wolters Kluwer.

Pincus, L. B., & McCalmont, T. H. (2019). Diseases of the skin. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed.). McGraw-Hill Education.

Zuberbier, T., Ansari, Z. A. H., Abdul Latiff, A. H., Abuzakouk, M. M., Agcaoili-De Jesus, M. S., Agondi, R. C., Al-Ahmad, M., Alangari, A. A., Alhameli, H., Alonso Bello, C. D., Alshareef, S., Al-Tamemi, S., Altrichter, S., Al Wahshi, H., Aquilina, S., Araújo, M., Arnaout, R., Asero, R., Ballmer-Weber, B., . . . Giménez-Arnau, A. M. (2026). The international guideline for the definition, classification, diagnosis and management of urticaria. Allergy, 81(8), 2582–2632. https://doi.org/10.1111/all.70210