Allergic contact dermatitis

Overview

•ACD = Type IV (delayed-type) hypersensitivity - T-cell mediated, no antibodies involved

•Sensitisation phase (clinically silent): allergen → Langerhans cell antigen presentation → memory T-cells generated

•Elicitation phase: re-exposure → memory T-cell activation → cytokine release → eczematous rash after 24-72 hours

•Patient may use a product for months/years before sensitisation - absence of previous reaction does NOT exclude ACD

•Common allergens: nickel (most common - jewellery, watches, belt buckles), rubber/latex (condoms, gloves), paraphenylenediamine/PPD (hair dye, black henna), fragrances, acrylates, preservatives (methylisothiazolinone), topical medications (neomycin, lanolin)

💡

Latex condoms are an important cause of pruritus vulvae - contact dermatitis is the most common cause of pruritus vulvae overall.

Presentation

•Intense pruritus with eczematous rash (erythema, vesicles, weeping acutely; fissuring and lichenification chronically)

•Delayed onset - rash 24-72 hours after re-exposure (not immediate)

•Distribution matches allergen contact - e.g. wrist under watchstrap (nickel), waistband, neck under necklace

•Can spread beyond exact contact site (unlike irritant contact dermatitis)

•Improves away from trigger - e.g. better at weekends or on holiday away from workplace

Investigations

🏆 Gold standard

•patch testing - allergen panels applied to upper back under occlusive patches for 48 hours; read at 48 hours and again at 96 hours/day 7; positive result = localised eczematous reaction at allergen site

🎯

Patch testing (Type IV, T-cell) is NOT the same as skin prick testing (Type I, IgE-mediated). Serum IgE and RAST are not useful for ACD - ACD is not antibody-mediated.

🥈 Second-line

•skin biopsy - only if diagnosis uncertain (e.g. to exclude psoriasis or cutaneous T-cell lymphoma)

Management

🥇 First-line

•allergen avoidance - substitute products (nickel-free jewellery, non-latex condoms, fragrance-free toiletries); improvement after avoidance may take 8-12 weeks

•Emollients and soap substitutes - restore epidermal barrier; used liberally and as soap replacement

•Topical corticosteroids - mild (hydrocortisone 1%) for face/flexures; moderate-potent (betamethasone valerate 0.1%) for hands/body

🥈 Second-line

•oral antihistamines - non-sedating (cetirizine 10 mg OD) for daytime itch; sedating (chlorphenamine) if sleep disturbed - treat symptom of itch only, not underlying mechanism

•Referral to dermatology - for patch testing, severe/recalcitrant disease, or occupational dermatitis