Immediate reaction - occurs within minutes and up to 2 hours after exposure to allergen, IgE mediated

Pathophysiology

• IgE-mediated antibody response to external antigen
• Routes of exposure - skin contact, inhalation, ingestion and injection
• Sensation stage: production of specific IgE by B cells (helped by T cells) in response to initial allergen exposure, residual IgE antibodies bind to circulating mast cells via Fc receptors
• Allergic stage: on re-exposure to allergen, the allergen will bind to IgE coated mast cells → cell degranulation (release of histamine and other inflammatory mediators)
• Approx. 20% of young children with moderate-severe atopic eczema also have food allergy

Clinical features

• Urticaria - very itchy lesions which appear within 1 hour, lasts 2-6 hours (sometimes up to 24)
  • Also called hives, wheals, nettle rash
• Angioedema - localised swelling of subcutaneous tissue or mucous membranes, non-pitting oedema, not itchy (unless associated with urticaria)
• Wheezing/asthma
• Anaphylaxis - severe, life threatening, generalised or systemic hypersensitivity reaction
  • Airway - pharyngeal or laryngeal oedema
  • Breathing - bronchospasm (wheeze) with tachypnoea
  • Circulation - hypotension (+ collapse) and/or tachycardia
  • Other common features - urticaria, angioedema, allergic rhinitis, allergic conjunctivitis

Investigations

• History: most important, consistent reaction with every exposure
• Bloods: specific IgE (70-75% specificity and sensitivity)
  • Safest
• Skin prick/prick-prick testing
  • If specific IgE negative but history convincing (suspect false negative)
  • 90%+ specificity and sensitivity
  • Anaphylaxis risk
• Challenge test: if skin prick test negative but history very convincing
• Serum mast cell tryptase level: to confirm anaphylaxis

Management

Long term management