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InflammatoryType IV hypersensitivityICD-10 L23.x

Allergic contact dermatitis

ACD · type IV hypersensitivity dermatitis · contact allergy

Allergic contact dermatitis is a delayed type-IV hypersensitivity reaction to a topical sensitiser, mediated by hapten-specific memory T cells. UK common allergens include nickel, fragrance mix, cobalt, paraphenylenediamine (PPD), Compositae mix, Myroxylon pereirae (balsam of Peru), preservatives (methylisothiazolinone, formaldehyde, parabens, quaternium-15) and topical antibiotics (neomycin, bacitracin). Skin-oncology relevance: post-Mohs / wound-dressing reactions, topical-therapy allergens (5-FU, imiquimod, lidocaine), and ICI-related contact-like exacerbations. Patch testing remains the diagnostic standard.

CurrentLast reviewed 16 May 2026
Clinical image of Allergic contact dermatitis
Allergic contact dermatitis. Image sourced from DermNet New Zealand. Used under CC BY-NC-ND 4.0. No endorsement implied.

Pathogenesis

  • Type-IV delayed hypersensitivity; CD4+ / CD8+ T-cell mediated.
  • Two phases:
    1. Sensitisation: hapten binds protein → Langerhans cell presentation → T-cell memory (10-14 days).
    2. Elicitation: re-exposure → eczematous reaction at site within 48-72 hours.
  • Common UK allergens (BSCA standard series): nickel, cobalt, chromium, fragrance, balsam of Peru, PPD, preservatives, rubber chemicals, antibiotics, plant allergens.

Clinical features

  • Eczematous pruritic erythema with vesicles / bullae in acute phase; lichenification chronic.
  • Geometric / linear distribution in keeping with contactant.
  • Common patterns:
    • Eyelid: nail varnish (via fingers), preservatives in cosmetics, latex.
    • Earlobe / neck: nickel from jewellery.
    • Wrist: watch strap nickel / leather chromium.
    • Hands: occupational; latex, rubber accelerators, preservatives.
    • Feet: rubber, leather chromium.
    • Genital / perianal: preservatives, fragrance.
    • Post-Mohs / wound: dressing adhesives (acrylates), antibiotic ointment, lidocaine.
  • Onset 24-72 hours after exposure; chronicity if exposure ongoing.

Patch testing

  • Patch testing: gold standard diagnosis.
  • British standard series (40-50 allergens); plus extended series tailored to occupation, body site or suspected sensitiser.
  • Read at 48 hours (D2), 96 hours (D4); some allergens require 7-day read.
  • Grading (ICDRG):
    • ?+ (doubtful), + (weak), ++ (strong), +++ (extreme), IR (irritant).
  • Patient's own products may be patch-tested ("as is" or diluted appropriately).
  • Refer to UK BSCA contact-dermatitis centres for specialist patch testing.

Differentials

  • Irritant contact dermatitis — non-immunologic; immediate burn-like; occupational hand wash.
  • Atopic eczema — flexural, atopic background.
  • Discoid eczema — round patches.
  • Tinea — annular advancing edge.
  • Phytophotodermatitis — UV-dependent streaky pattern.
  • Drug eruption — systemic exposure.
  • ICI-related eczematous dermatitis.

Management

  • Identify and avoid the allergen — patch testing followed by structured allergen-avoidance counselling and substitute products.
  • Topical: mid-to-high-potency corticosteroid 2-3 weeks; topical calcineurin inhibitors for face / flexures.
  • Oral: prednisolone short course for severe flare; sedating antihistamine for itch.
  • Phototherapy (NBUVB / PUVA) for chronic / refractory dermatitis.
  • Systemic immunosuppression: methotrexate, azathioprine, mycophenolate, ciclosporin in severe / occupational-related disability.
  • Dupilumab being evaluated; case series suggest benefit in some severe ACD.
  • Occupational: consider Health and Safety Executive (HSE) reporting; protective clothing; gloves (nitrile preferred); skin care regime.
  • Counsel: persistent dermatitis up to weeks after allergen-removal; chronic ACD has 30-50% long-term persistence.

References

  1. Johansen JD et al. European Society of Contact Dermatitis guideline for diagnostic patch testing — recommendations on best practice. Contact Dermatitis. 2015;73:195-221.
  2. British Society of Cutaneous Allergy (BSCA). British baseline series 2019. London: BSCA; 2019.
  3. Brasch J et al. Guideline contact dermatitis: S1-Guidelines of the German Contact Allergy Group. Allergo J Int. 2014;23:126-138.
  4. Bourke J et al. Guidelines for the management of contact dermatitis: an update. Br J Dermatol. 2009;160:946-954.

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