Causes of Urticaria
Urticaria results from mast cell and basophil activation with subsequent release of histamine and other vasoactive mediators, triggered by immunologic, non-immunologic, physical, or idiopathic mechanisms. 1
Immunologic Causes
IgE-Mediated Allergic Reactions
- Environmental allergens (latex, nuts, fish) represent the classic allergic pathway, where IgE antibodies bound to mast cell receptors are triggered by specific allergens, particularly in acute urticaria 2, 1
- Food allergies cause acute urticaria more commonly in children than adults, though the actual confirmed rate in adults is below 1% despite 63% of patients suspecting food as the trigger 3
- Insect stings can trigger IgE-mediated urticaria 4
Autoimmune Mechanisms
- Circulating IgG autoantibodies directed against the high-affinity IgE receptor (FcεRIα) or against IgE itself are present in 40-60% of patients with chronic urticaria 4, 5
- Thyroid autoimmunity is present in 14% of chronic urticaria patients, with elevated antithyroid antibodies (antithyroglobulin and antiperoxidase) even in euthyroid individuals 1, 4
Non-Immunologic Causes
Direct Mast Cell Degranulation
- Certain drugs directly trigger mast cell degranulation without requiring IgE, including codeine, opioids, and radiocontrast media 1
- NSAIDs and aspirin cause or aggravate urticaria through mechanisms probably involving leukotriene formation and histamine release, accounting for approximately 9% of acute urticaria cases 1, 3
Infectious Triggers
- Infections are among the most common causes of acute urticaria, particularly viral infections 6, 7
- Helicobacter pylori shows some association, with resolution of chronic urticaria more likely when antibiotic therapy successfully eradicates the infection 2, 1
- Proposed associations with occult infections (dental abscess, gastrointestinal candidiasis) have little supporting evidence 2, 1
Physical Urticaria Causes
Mechanical Stimuli
- Pressure, vibration, and dermographism activate mast cells through direct physical forces 1
- Physical factors account for 5-10% of chronic urticaria cases in children 6
Thermal Stimuli
- Heat and cold trigger temperature-sensitive pathways 1
- Cold-induced familial urticaria represents a congenital form 6
Additional Triggers and Associations
Chemical and Environmental Factors
- Contact with chemicals and irritants can trigger acute episodic urticaria 6
- Naturally occurring pseudoallergens in food (not just artificial additives) cause reactions in more than 55% of patients with chronic urticaria responsive to low-pseudoallergen diets 3
Systemic Disease Associations
- Urticaria can be a symptom of systemic diseases including collagenopathies, endocrinopathies, tumors, hemolytic diseases, and celiac disease 6
- No statistical association exists between malignancy and urticaria, despite individual case reports 2, 1
Idiopathic Urticaria
- In many instances of acute urticaria, no specific cause can be identified (idiopathic urticaria) 6
- The causative factor for chronic urticaria is identified in only about 20% of cases 6
Important Clinical Caveats
- Aggravating factors that worsen existing urticaria include overheating, stress, and alcohol, though these are not primary causes 2
- ACE inhibitors should be avoided in patients with angioedema without wheals and used with caution when angioedema accompanies urticaria 2
- Oestrogens should be avoided in hereditary angioedema 2