Allergic Contact Dermatitis

What Is It?

Allergic contact dermatitis (ACD) is an immune-mediated skin reaction that occurs when your skin comes into contact with a substance your immune system has become sensitized to. Unlike food or airborne allergies (which are IgE-mediated), ACD is a delayed hypersensitivity reaction — it typically takes 24–72 hours after contact for the rash to develop.

What Does It Look Like?

• Red, itchy rash — sometimes with blisters or weeping
• Rash appears in the pattern of where contact occurred (e.g., earlobe from earrings, wrist from watch strap)
• Can spread beyond the contact site in severe cases
• Chronic cases may show dry, thickened, or cracked skin
• Often confused with eczema (atopic dermatitis)

Common Causes

• Metals: Nickel (most common), cobalt, chromate — found in jewelry, belt buckles, snaps
• Fragrances: Found in perfumes, lotions, cosmetics, household cleaners
• Preservatives: Formaldehyde releasers, isothiazolinones (in wet wipes, shampoos)
• Rubber accelerators: Thiuram mix, carba mix — found in latex gloves, elastic
• Hair dye chemicals: PPD (p-phenylenediamine), ammonium persulfate
• Topical medications: Neomycin, bacitracin, benzocaine
• Plants: Poison ivy/oak (urushiol), compositae plants

How Is It Diagnosed?

The gold standard for diagnosing ACD is patch testing, performed by a dermatologist. Small amounts of common allergens are applied to the back under occlusive tape for 48 hours, then read at 48 and 96 hours. A positive reaction indicates true sensitization to that allergen.

Treatment

• Avoidance: The most important step — identify and eliminate the allergen
• Topical corticosteroids for active flares
• Topical calcineurin inhibitors (tacrolimus, pimecrolimus) for maintenance
• Cool compresses for symptomatic relief
• Oral antihistamines may reduce itch (though they don't treat the underlying reaction)
• Severe cases may require short courses of oral steroids