Urticaria and Angioedema

• IgE-mediated mechanisms (Type I hypersensitivity): Cross-linking of high-affinity IgE receptors (FcεRI) on mast cells and basophils by multivalent antigens triggers rapid degranulation within minutes. This mechanism accounts for most acute urticaria cases. The cross-linking initiates a signaling cascade through Lyn and Syk tyrosine kinases, leading to increased intracellular calcium and activation of protein kinase C. Preformed mediators including histamine, tryptase, and heparin are released from cytoplasmic granules within seconds to minutes, producing immediate symptoms. Histamine binds to H1 and H2 receptors on endothelial cells, increasing intracellular cAMP and cGMP through different G-protein coupled signaling pathways, resulting in endothelial cell contraction and intercellular gap formation. Simultaneously, newly synthesized mediators including leukotrienes (particularly LTC4, LTD4, LTE4) and prostaglandins are generated from arachidonic acid via the 5-lipoxygenase and cyclooxygenase pathways, respectively, amplifying vasodilation and vascular permeability. Additionally, platelet-activating factor (PAF) contributes to leukocyte recruitment and inflammatory amplification.

• Non-IgE-mediated (complement-dependent) mechanisms: In some cases, particularly drug reactions and transfusion reactions, complement activation (either classical or alternative pathway) directly activates mast cells through C3a and C5a anaphylatoxins binding to complement receptors (C3aR, C5aR) on mast cell surfaces, bypassing the need for IgE. This pathway is particularly important in acquired C1-inhibitor deficiency and certain hereditary angioedema (HAE) variants where deficient or dysfunctional C1-inhibitor leads to uncontrolled activation of the contact system, generating excessive kallikrein and bradykinin. Unlike histamine-mediated reactions, bradykinin-induced angioedema is responsive to bradykinin B2 receptor antagonists but resistant to antihistamines and corticosteroids.

• Direct mast cell activation (non-immunologic): Certain substances including physical stimuli (pressure, cold, vibration, heat), exercise, stress, and drugs like NSAIDs, opioids, and ACE inhibitors can directly activate mast cells or alter the threshold for degranulation without immunologic trigger. Physical urticaria results from direct activation pathways or altered mast cell sensitivity. The molecular mechanism often involves altered calcium signaling or increased expression of IgE receptors on mast cell surfaces.

• Bradykinin-mediated angioedema pathways: The contact system (intrinsic coagulation cascade) is central to hereditary angioedema types 1 and 2 (C1-INH deficiency) and type 3 (normal C1-INH) variants. Factor XII (Hageman factor) initiates a cascade through prekallikrein to kallikrein, which cleaves high-molecular-weight kininogen (HMWK) to generate bradykinin, a potent vasodilator and mediator of vascular permeability. In normal individuals, C1-inhibitor (C1-INH) tightly regulates this cascade by inhibiting activated Factor XII, plasma kallikrein, and Factor XI. Deficiency or dysfunction of C1-INH removes this brake, leading to persistent bradykinin generation and recurrent episodes of angioedema. Bradykinin acts through B2 receptors on endothelial cells and tissues to increase vascular permeability, cause pain (distinctive feature of angioedema), and trigger inflammation. This explains why hereditary and acquired angioedema present with deeper, non-pitting edema that does not respond to antihistamines or corticosteroids.

• Chronic urticaria pathophysiology: In chronic idiopathic urticaria, multiple mechanisms contribute: (1) autoimmune mechanisms with functional autoantibodies against FcεRI or IgE itself found in ~40-50% of patients, perpetuating mast cell activation; (2) impaired regulatory T cell (Treg) function reducing immune tolerance; (3) altered mast cell biology with increased releasability and reduced apoptosis; (4) dysregulated innate immunity with increased Th2 cytokines (IL-4, IL-5) and reduced Th1 responses; (5) microbiome alterations potentially affecting immune tolerance; and (6) genetic susceptibility with polymorphisms in genes regulating mast cell function and IgE regulation. The distinction between acute and chronic urticaria has important pathophysiologic implications: acute urticaria typically resolves as the trigger is eliminated or antibodies clear, while chronic urticaria reflects persistent dysregulation of mast cell-mediated inflammation requiring sustained suppression rather than elimination of a specific trigger.

• IgE-mediated food allergies: Food proteins (peanuts, tree nuts, shellfish, milk, eggs, wheat, soy) are the most common triggers of acute urticaria in children and young adults. Reactions typically occur within minutes to 2 hours of exposure. The risk is highest in patients with atopic history and is associated with other atopic conditions. Cross-reactivity between related foods can occur (e.g., shellfish cross-reactivity, tree nut cross-reactivity).

• Medication reactions: Beta-lactam antibiotics (penicillins, cephalosporins) are among the most common drug triggers, occurring via IgE-mediated mechanisms in true allergic patients (1-3% incidence with penicillins). NSAIDs trigger urticaria through both immunologic and non-immunologic pathways, with aspirin sensitivity being a classic trigger in some patients (particularly those with concurrent rhinosinusitis and asthma—Samter's triad). ACE inhibitors cause urticaria and angioedema in 0.1-0.2% of users, with higher incidence in Black patients; the mechanism involves accumulation of bradykinin due to inhibition of ACE (kininase II). Opioids and vancomycin cause direct mast cell activation. Sulfonamides, barbiturates, and antiepileptics (particularly aromatic compounds like phenytoin, carbamazepine) trigger reactions in 1-3% of exposed patients.

• Infections: Viral infections (especially rhinovirus, coronavirus, hepatitis B, hepatitis C, and EBV) are the most common triggers of acute urticaria in adults; the rash typically appears during or shortly after symptomatic infection. Bacterial infections (particularly sinusitis, otitis, and urinary tract infections) and parasitic infections (helminths in endemic areas) can trigger urticaria through immune complex formation or direct antigenic stimulation. Streptococcal pharyngitis is a classic bacterial trigger, particularly relevant in pediatric populations.

• Environmental and occupational exposures: Inhaled allergens (pollen, animal dander, dust mites) occasionally trigger urticaria but more commonly cause rhinitis or asthma. Latex exposure (particularly in healthcare workers) causes IgE-mediated reactions. Occupational chemicals and contact allergens can trigger localized or generalized reactions depending on the nature and extent of exposure. Insect stings/bites (particularly Hymenoptera: bees, wasps, hornets) cause acute IgE-mediated reactions and are a medical emergency if associated with systemic symptoms.

• Physical stimuli: Dermographism (symptomatic dermographism) affects 4-5% of the population and is triggered by mechanical trauma or stroking of the skin. Cold urticaria (1-3% prevalence) is triggered by exposure to cold temperatures or cold objects; some cases are associated with cryoglobulins or cryofibrinogenemia. Heat urticaria is less common but can be triggered by warm environments. Solar urticaria is rare and involves UV radiation triggering mast cell activation. Cholinergic urticaria is triggered by elevation in core body temperature (exercise, hot environments, emotional stress) and presents with characteristic small wheals (1-3 mm) and intense pruritus. Delayed pressure urticaria develops 4-12 hours after sustained pressure application and can persist for days, likely mediated by different inflammatory cascades than other physical urticarias.

• Chronic idiopathic urticaria (CIU): By definition, no identifiable external trigger can be documented despite thorough investigation in ~80-90% of chronic urticaria cases. However, autoimmune mechanisms (functional autoantibodies against FcεRI or IgE) are demonstrable in 40-50% of CIU patients, particularly those with more severe disease and poor response to antihistamines. Thyroid autoimmunity is found in 15-20% of CIU patients, though causality is disputed. Helmintic infections (in endemic regions) and Helicobacter pylori infection have been implicated in some studies but evidence remains controversial. Systemic diseases including systemic lupus erythematosus (SLE), Sjögren syndrome, antiphospholipid syndrome, and lymphoproliferative disorders can present with chronic urticaria. Mast cell disorders (systemic mastocytosis, clonal mast cell activation) should be considered in refractory cases.

• Hereditary angioedema (HAE): Type 1 (85% of HAE cases): Quantitative C1-inhibitor deficiency due to mutations in the SERPING1 gene encoding C1-inhibitor protein. Type 2 (10-15% of cases): Qualitatively abnormal (dysfunctional) C1-inhibitor despite normal levels. Type 3 (5% of cases): Normal C1-inhibitor levels and function but associated with mutations in Factor XII or Angiopoietin-1 (ANGPT1) genes, causing enhanced contact system activation through alternative mechanisms. Type 3 HAE may also include cases with mutations in Plasminogen or Kallikrein genes. HAE is inherited in an autosomal dominant pattern with variable penetrance; approximately 25% of cases are de novo mutations.

• Acquired C1-inhibitor deficiency: Acquired C1-INH deficiency (ACEI) can occur secondary to lymphoproliferative disorders (B-cell lymphomas, CLL) where monoclonal B cells produce antibodies against C1-INH or consume it, or secondary to underlying autoimmune diseases. Typically occurs in older patients (>40 years) without family history, distinguishing it from hereditary forms.

• Other systemic triggers: Transfusions and contrast media reactions trigger urticaria through complement activation (particularly classical pathway). Idiopathic and hormonal factors: In women, some cases of chronic urticaria are hormone-dependent, worsening with oral contraceptives, hormone replacement therapy, or menstrual cycle changes. Stress and psychological factors can exacerbate existing urticaria through increased mast cell releasability mediated by neuropeptides (substance P, calcitonin gene-related peptide) and enhanced TH2 responses.

• Pruritus: Often the most bothersome symptom for patients, ranging from mild to severe and sometimes intolerable, potentially disrupting sleep and quality of life. Pruritus precedes the appearance of wheals by minutes to hours in many cases. The itch is mediated primarily by histamine binding to H1 receptors on sensory nerve endings, though other mediators including leukotrienes and neuropeptides contribute. In some cases (particularly hereditary angioedema), pruritus is notably absent, which is a valuable diagnostic clue.

• Wheals (hives): The pathognomonic lesion consists of well-demarcated, raised, erythematous or pale plaques typically 2-10 mm in diameter, though larger lesions (up to several centimeters) can coalesce. Wheals are transient, typically resolving completely within 24 hours without any residual marks, scarring, or pigment changes (if individual lesions persist >24 hours, consider urticarial vasculitis or other diagnosis). Lesions may appear in crops, with new lesions developing as others resolve. The central pale appearance with surrounding erythematous flare reflects increased vascular permeability and localized vasodilation. Wheals are often described as having a "bizarre" or changing distribution.

• Angioedema: Characterized by deeper dermal and submucosal edema, appearing as well-demarcated, non-pitting swelling often affecting the face (lips, tongue, eyelids), throat, hands, feet, and occasionally genitalia or gastrointestinal tract. Angioedema is typically less pruritic than urticaria and may be painful or burning rather than itchy, especially in bradykinin-mediated forms (hereditary and acquired C1-INH deficiency). Lesions are skin-colored or slightly erythematous (less red than urticaria). Notably, individual angioedema lesions can persist for 24-72 hours, longer than urticaria, and may appear to move to different sites as inflammation resolves in one area and develops in another. Angioedema without concurrent urticaria is an important clinical pattern suggesting C1-INH deficiency rather than IgE-mediated disease.

• Mucosal involvement: Laryngeal angioedema is a medical emergency, presenting with dyspnea, stridor, hoarseness, or sensation of throat tightness. Involvement of the gastrointestinal mucosa causes abdominal pain, nausea, vomiting, and diarrhea due to mucosal swelling and increased peristalsis. These manifestations are particularly common in hereditary and acquired C1-INH deficiency and warrant immediate evaluation and potential airway protection.

• Constitutional symptoms: Fever and systemic symptoms are typically absent in urticaria and angioedema; their presence should prompt evaluation for alternative diagnoses (urticarial vasculitis, serum sickness, other systemic diseases). However, patients may report fatigue and malaise related to pruritus-induced sleep disruption and psychological stress.

• Physical exam findings in urticaria:
• Individual wheals: Well-demarcated, raised, erythematous or pale lesions with surrounding flare
• Distribution: Can be generalized or localized; commonly affects exposed areas and pressure points but can appear anywhere
• Diascopy (application of glass slide pressure): Wheals typically blanch completely with pressure, distinguishing them from non-blanching rashes
• Dermographism (white dermographism): Transient blanching of normal skin after stroking due to increased vascular permeability in response to minor trauma; present in 4-5% of population but exaggerated in some urticaria patients
• Red dermographism: Erythema (rather than blanching) after skin stroking, seen in some patients
• Urticaria factitia or symptomatic dermographism: When scratching or stroking produces wheals (distinct from simple dermographism)
• Absence of scale, vesicles, or other primary skin lesions: Their presence indicates alternative diagnosis

• Physical exam findings in angioedema:
• Non-pitting, localized swelling: Well-demarcated but with less distinct borders than urticaria
• Skin-colored or mildly erythematous appearance:

Initial approach

• Acute urticaria (<6 weeks): no routine testing. Targeted IgE testing (skin prick or serum specific IgE) only when history points to a discrete food, drug, or Hymenoptera trigger.
• Chronic spontaneous urticaria (>6 weeks): limited screen only — CBC with differential and an inflammatory marker (ESR or CRP). Elevated inflammatory markers or eosinophilia argue against simple CSU. Anti-thyroid peroxidase antibodies and total IgE are optional and mainly prognostic.
• Inducible urticaria: confirm with provocation — stroking the skin (symptomatic dermographism), the ice cube test for cold urticaria, warm bath or exercise challenge for cholinergic urticaria, controlled light exposure for solar urticaria.

Angioedema without wheals — the complement workup

• C4 is the best initial screen: low both during and between attacks in C1-INH deficiency; a normal C4 during an attack makes HAE types 1/2 very unlikely.
• C1-INH antigenic level and functional assay are confirmatory: type 1 = low level and low function; type 2 = normal or high level with low function.
• C1q separates hereditary (normal C1q) from acquired C1-INH deficiency (low C1q, older patient, lymphoproliferative disease).
• Normal C4, C1-INH level, and function with a compelling family history → genetic testing (F12, PLG, ANGPT1).

Histaminergic urticaria — stepwise per the AAAAI/ACAAI practice parameter and the EAACI/GA²LEN/EuroGuiDerm/APAAACI guideline

• Second-generation H1 antihistamines (cetirizine, levocetirizine, fexofenadine, loratadine) at standard dose are first-line; they are non-sedating and lack anticholinergic burden.
• Updose the same agent up to fourfold before adding anything else — the single most tested escalation step.
• Add-on options: an H2 blocker (famotidine) or a leukotriene receptor antagonist (montelukast), both of modest benefit.
• Omalizumab (anti-IgE monoclonal antibody) is FDA-approved for antihistamine-refractory chronic spontaneous urticaria and is the preferred third step.
• Cyclosporine (calcineurin inhibitor) is reserved for omalizumab failures, with blood pressure and creatinine monitoring.
• Systemic corticosteroids (prednisone) only as a short burst for severe flares; chronic use is explicitly discouraged.

• Acute attacks: plasma-derived or recombinant C1-INH concentrate, the kallikrein inhibitor ecallantide, or the B2-receptor antagonist icatibant; treat laryngeal attacks early and secure the airway.
• Long-term prophylaxis: C1-INH concentrate, lanadelumab (anti-kallikrein antibody), or berotralstat (oral kallikrein inhibitor). Attenuated androgens (danazol) are legacy therapy.
• Contraindicated in HAE: ACE inhibitors and estrogen-containing contraceptives, both of which precipitate attacks. Fresh frozen plasma is a last resort only.

Emergencies

• Laryngeal angioedema with asphyxia: submucosal edema of the supraglottis obstructs airflow. Signaled by hoarseness, stridor, throat tightness, or drooling — these precede desaturation. Intubate early or perform surgical airway; delay is what kills. Bradykinin-mediated attacks progress over hours and do not reverse with epinephrine.
• Anaphylaxis: systemic mast cell degranulation producing hypotension, bronchospasm, and vomiting alongside hives. Urticaria plus any respiratory or cardiovascular sign is anaphylaxis until proven otherwise.
• HAE abdominal attack: bowel wall edema causes severe colicky pain, vomiting, and third-spacing with hypovolemia; CT may show bowel wall thickening and ascites. The classic complication is unnecessary exploratory laparotomy in an undiagnosed patient.

Disease-related

• Sleep deprivation, anxiety, and depression from unrelenting pruritus — quality-of-life impairment in chronic urticaria rivals that of ischemic heart disease.
• Excoriation with secondary bacterial infection from scratching.
• Hypocomplementemic urticarial vasculitis: immune-complex deposition may extend to glomerulonephritis, obstructive lung disease, and uveitis; suspect when wheals bruise and last beyond a day.

Treatment-related

• First-generation antihistamines (diphenhydramine, hydroxyzine): central H1 blockade and anticholinergic effect cause sedation, impaired driving, urinary retention, and delirium in older adults — listed as potentially inappropriate by the AGS Beers Criteria.
• Systemic corticosteroids: hyperglycemia, HPA-axis suppression, bone loss, and rebound flare on taper.
• Cyclosporine: afferent arteriolar vasoconstriction → hypertension and rising creatinine; also gingival hyperplasia and hirsutism.
• Omalizumab: carries an FDA boxed warning for anaphylaxis, which may be delayed after injection; observation after early doses is standard.
• Attenuated androgens (danazol): hepatotoxicity, hepatic adenoma, dyslipidemia, virilization; contraindicated in pregnancy and in children.
• Epinephrine: tachyarrhythmia and myocardial ischemia in susceptible patients — still given, since untreated anaphylaxis is more dangerous.

• Wheals lasting >24 hours, painful or burning, leaving bruising or hyperpigmentation = urticarial vasculitis, not urticaria. Best next step: punch biopsy plus C3/C4. This is the single most reliable morphology question on the exam.
• Angioedema without hives and without pruritus = bradykinin, not histamine. Best next step: serum C4 as the screening test, then C1-INH antigen and function. C1q is the discriminator — normal in hereditary, low in acquired C1-INH deficiency with underlying lymphoproliferative disease.
• Antihistamines, corticosteroids, and epinephrine do not reverse HAE attacks. Give C1-INH concentrate, icatibant, or ecallantide. A stem showing a young patient with recurrent facial swelling and abdominal pain who "failed Benadryl and prednisone" is HAE.
• ACE-inhibitor angioedema can begin months to years after starting the drug — a normal prior tolerance period does not exclude it. Tongue and lip swelling, disproportionately common in Black patients. Stop the drug permanently.
• Escalation in chronic spontaneous urticaria: second-generation H1 antihistamine → updose up to fourfold → omalizumab → cyclosporine, per the AAAAI/ACAAI practice parameter. Chronic systemic steroids are the wrong answer.
• Do not order a broad allergy or autoimmune panel for chronic urticaria. CBC with differential and ESR/CRP suffice; extensive testing is the classic distractor.
• Cold urticaria: confirm with the ice cube test; warn about swimming, where whole-body cooling can cause systemic mast cell release and drowning. Check for cryoglobulins when history suggests it.
• Cholinergic urticaria: tiny 1–3 mm punctate wheals with large flares triggered by a rise in core body temperature, not by sweat contact — distinguish from exercise-induced anaphylaxis, which involves hypotension.