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Explain the mechanism of urticaria

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urticaria mast cell degranulation IgE mechanism diagram

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Mechanism of Urticaria

Urticaria (hives) is fundamentally a disorder of cutaneous mast cell degranulation leading to transient dermal microvascular hyperpermeability, plasma extravasation, and sensory nerve activation — producing the classic wheal-and-flare response with pruritus. — Fitzpatrick's Dermatology, Robbins & Cotran Pathologic Basis of Disease

The Central Pathway: Mast Cell Degranulation

Dermal mast cells are the central effectors. When activated, they release:
Preformed (granule) mediators — released within minutes:
  • Histamine — binds H1 receptors on postcapillary venule endothelium → vasodilation and increased vascular permeability → wheal; binds sensory nerves → pruritus
  • Tryptase, chymase — serine proteases that amplify inflammation
Newly synthesized mediators — generated via arachidonic acid:
  • Prostaglandins (PGD₂) — vasodilation, itch
  • Leukotrienes (LTC₄, LTD₄) — potent vasodilators and bronchoconstrictors
  • Platelet-activating factor (PAF) — increases vascular permeability
Cytokines (TNF-α, IL-4, IL-5, IL-13) — sustain the inflammatory response
The net result: vasodilation + plasma extravasation into the dermis = wheal (edema); surrounding erythema from arteriolar vasodilation (flare); pruritus from direct sensory nerve stimulation by histamine and kinins. — Rosen's Emergency Medicine

Classification by Mechanism

1. Mast Cell-Dependent, IgE-Dependent (Type I Hypersensitivity)

This is the classic allergic mechanism:
  1. Sensitization phase: On first antigen exposure (pollen, food, drugs, insect venom), Th2 cells produce IL-4 and IL-13, driving B-cell class switching → antigen-specific IgE is produced and binds to high-affinity FcεRI receptors on dermal mast cells and circulating basophils
  2. Elicitation phase: Re-exposure to the same antigen cross-links adjacent FcεRI-bound IgE molecules, triggering a signaling cascade:
    • Activation of SYK → LAT → PLCγ → PKC
    • Rise in intracellular Ca²⁺
    • Explosive mast cell degranulation with release of histamine and other mediators
Symptoms typically develop within 15 minutes to 1 hour of antigen exposure. Despite being the best-understood mechanism, classical type I allergy accounts for less than 10% of all urticaria cases. — Fitzpatrick's Dermatology
Mast cell and basophil activation by autoantibodies showing IgE, FcεRI, autoantibodies, C5a, and omalizumab interactions
Figure: The pathogenesis of autoimmune urticaria and mechanism of action of omalizumab — Fitzpatrick's Dermatology

2. Autoimmune Mechanism (IgE-Dependent variant — Chronic Spontaneous Urticaria)

One-third to one-half of chronic spontaneous urticaria (CSU) cases have a detectable autoimmune basis:
  • Anti-FcεRI IgG autoantibodies: Directly bind and cross-link the high-affinity IgE receptor on mast cells/basophils → degranulation (complement-dependent, via C5a)
  • Anti-IgE IgG autoantibodies: Cross-link IgE itself while it is bound to FcεRI → same downstream result
  • These can be detected by the autologous serum skin test (ASST) and the basophil histamine release test
  • The complement fragment C5a amplifies degranulation
This explains why omalizumab (anti-IgE monoclonal antibody) works in CSU: by sequestering free IgE, it depletes IgE available to bind FcεRI, gradually downregulating FcεRI expression on mast cells, reducing their reactivity. — Fitzpatrick's Dermatology
Chronic spontaneous urticaria — IgE-dependent vs IgE-independent pathways

3. Mast Cell-Dependent, IgE-Independent

Certain substances directly trigger mast cell degranulation without prior IgE sensitization:
  • Opiates (morphine, codeine), vancomycin, radiocontrast media — direct mast cell degranulation via MRGPRX2 receptor
  • Foods (lobster, strawberries) — direct histamine release, non-immunologic
  • No prior sensitization required; can occur on first exposure

4. Mast Cell-Independent, IgE-Independent

These mechanisms bypass mast cells altogether:
  • Aspirin/NSAIDs: Inhibit cyclooxygenase (COX-1) → shunt arachidonic acid toward the lipoxygenase pathway → excess leukotrienes → vascular permeability. The precise mechanism is not fully elucidated, and effects can persist weeks after ingestion
  • Hereditary angioedema (HAE): Deficiency of C1 inhibitor → unregulated activation of the early complement cascade and contact activation system → generation of bradykinin → binds B2 receptors on postcapillary venule endothelium → vasodilation and edema (notably mast cell-independent; antihistamines are ineffective in pure HAE)
  • ACE inhibitors: Block bradykinin degradation → bradykinin accumulation → angioedema (same pathway as HAE)
— Robbins & Cotran Pathologic Basis of Disease

5. Neurogenic Mechanism

Cutaneous mast cells express receptors for numerous neuropeptides — substance P, vasoactive intestinal peptide (VIP), somatostatin, neurokinin A/B, CGRP, ACTH, and acetylcholine. In cholinergic urticaria, excessive acetylcholine release from cholinergic nerve terminals (in response to heat/exercise/sweating) directly activates mast cells. Two thirds of cholinergic urticaria patients are also sensitized to antigens in human sweat (specifically MGL_1304, a protein from Malassezia globosa), explaining the IgE-mediated overlap. — Fitzpatrick's Dermatology

Summary Table

MechanismKey MediatorExample TriggersIgE?Mast Cell?
Type I hypersensitivityHistamine, leukotrienesFoods, drugs, insect venom✅ Yes✅ Yes
Autoimmune (anti-FcεRI/anti-IgE IgG)Histamine, C5aIdiopathic CSU✅ Yes✅ Yes
Direct mast cell degranulationHistamineOpiates, contrast media❌ No✅ Yes
COX inhibition (aspirin)LeukotrienesNSAIDs❌ No✅ Yes
Bradykinin (HAE/ACEi)BradykininC1-inh deficiency, ACEi❌ No❌ No
NeurogenicHistamine, acetylcholineHeat, exercise, sweat±✅ Yes

Histopathology

Regardless of mechanism, the histological features of urticaria are subtle:
  • Widened collagen bundles (dermal edema)
  • Dilated dermal lymphatics
  • Sparse superficial perivenular infiltrate: mononuclear cells, rare neutrophils, ± eosinophils
  • No epidermal changes (this distinguishes urticaria from eczema)
— Robbins & Cotran Pathologic Basis of Disease
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