From the Guidelines
Urticaria is primarily a Type I hypersensitivity reaction. In this reaction, allergens cross-link IgE antibodies on mast cells and basophils, triggering the release of histamine and other inflammatory mediators 1. These chemicals cause the characteristic wheals, erythema, and pruritus seen in urticaria by increasing vascular permeability and causing vasodilation. The reaction typically develops rapidly after exposure to the triggering allergen and can resolve within hours.
Key Characteristics of Urticaria
- Urticaria often remains idiopathic after allergic, infectious, physical, and drug-related causes have been excluded as far as possible 1.
- At least 30% of patients with the ordinary presentation of chronic urticaria appear to have an autoimmune aetiology 1.
- The term urticaria is widely used to describe an eruption of weals, and it is now also increasingly being used to define a disease characterized by short-lived itchy weals, angio-oedema, or both together 1.
Treatment and Management
- Treatment focuses on antihistamines, particularly H1-receptor antagonists like cetirizine, loratadine, or fexofenadine at standard doses.
- For acute severe cases, short courses of oral corticosteroids may be needed.
- Identifying and avoiding triggers is essential for long-term management.
- In chronic urticaria, which lasts more than six weeks, autoimmune mechanisms may play a role, and additional therapies like omalizumab might be considered 1.
Clinical Classification of Urticarias
- Ordinary urticaria: Acute (up to 6 weeks of continuous activity), Chronic (6 weeks or more of continuous activity), Episodic (acute intermittent or recurrent activity) 1.
- Physical urticarias: Mechanical, Delayed pressure urticaria, Symptomatic dermographism, Vibratory angio-oedema, Thermal, Cholinergic urticaria, Cold contact urticaria, Localized heat urticaria 1.
Understanding urticaria as a Type I hypersensitivity reaction explains why antihistamines are the cornerstone of treatment, as they directly counteract the primary mediator responsible for symptoms.
From the Research
Types of Hypersensitivity Reactions
Urticaria can be classified into different types of hypersensitivity reactions, including:
- Type I hypersensitivity reaction: mediated by IgE antibodies, although this is not the primary mechanism in most cases of urticaria 2
- Type II hypersensitivity reaction: mediated by IgG auto-antibodies that bind to IgE or the IgE receptor on mast cells, accounting for up to 50% of chronic urticaria cases 2
- Type III hypersensitivity reaction: mediated by circulating immune complexes that bind to Fc receptors on mast cells 2
- Type IV hypersensitivity reaction: mediated by T-cells that induce mast cell activation and histamine release 2
Immunological and Nonimmunological Urticaria
Urticaria can be distinguished into immunological and nonimmunological types:
- Immunological urticaria: mediated by antibodies and/or T-cells that result in mast cell activation 2
- Nonimmunological urticaria: resulting from mast cell activation through membrane receptors involved in innate immunity or direct toxicity of xenobiotics 2
Mediators of Urticaria
Various mediators are involved in the pathogenesis of urticaria, including:
- Histamine: a major mediator of most forms of urticaria 3, 4
- Prostaglandins: contribute to the polymorphism of urticaria and its variable evolution 2
- Leukotrienes: involved in the pathogenesis of urticaria 2, 5
- Cytokines and chemokines: produced at different times after mast cell activation, contributing to the clinical spectrum of urticaria 2