Type I Allergy

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 presentation

  • 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

  1. Allergen avoidance
  1. Anti-histamines (first line after avoidance)
  1. Corticosteriods (second line)
  1. Others:
      • Adrenaline autoinjector for anaphylaxis
        • 300µg adults, 150µg children
        • All patients should be prescribed 2 pens
      • Mast cell stabilisers (e.g. sodium cromoglycate) can be useful for asthma in the context of allergy
      • Immunotherapy for some patients
      • Medic Alert bracelet

3 step plan for acute allergic reaction

  1. At first sign of reaction give chlorphenamine
  1. If allergic reaction does not resolve or gets worse over next 30 mins seek urgent medical help and give prednisolone
  1. If child becomes blue or collapses give adrenaline IV and call 999