30th July 2026

Allergic Contact Dermatitis

Background

Contact dermatitis also known as contact eczema, is an exogenous inflammatory skin disorder. The two main subtypes of the condition are Irritant contact dermatitis (ICD) and allergic contact dermatitis (ACD). ACD is a T-cell mediated type 4 hypersensitivity reaction triggered by exposure to an allergen hosts are previously sensitised to. It is a relatively common disease 

Epidemiological data on ACD is sparse and thus the incidence and prevalence is not known.

Aetiology

Allergens act as haptens (small hydrophobic chemicals) and bind to form a hapten-protien complex to become antigenic and penetrate the stratum corneum. The complex in engulfed by langarhans cells which migrate to lymph nodes, and present the antigen to naive CD4 T cells that proliferate into memory and effector T cells, causing sensitisation to the allergen. Re-exposure will lead to an inflammatory reaction after approximately 48-72 hours, presenting as a dermatitis. 

Risk factors

• Occupation: e.g. hairdressing, construction, chemical industry, healthcare professionals 
• Age: sensitisation occurs early in childhood
• History of atopy

Common allergens include: nickel, neomycin, myroxylon pereira, fragrance mx, thimerosal, sodium gold thiosulfate. Poison ivy is also a common cause. 

Clinical manifestations

Acute ACD

• Initially confined to site of contact, but diffuse reactions may occur
• Erythematous lesion
• Itch is the predominant symptom
• Burning in some cases 
• Oedema
• Blistering

Chronic ACD

• Dry, thick scale
• Hyperkeratosis
• Lichenification
• Fissures (cracking)

Differential diagnosis 

• Irritant contact dermatitis
• Atopic dermatitis
• Palmoplantar psoriasis 
• Contact urticaria
• Fungal infections e.g. Tinea corporis

Investigations

Patch testing: help differentiate between ACD and ICD

Medical management 

• Identification of trigger
• Avoidance of future exposure
  • Education
  • Minimise contact with allergen
  • Use of gloves
• Treatment of existing dermatitis 
  • Emollients: reduce transepidermal water loss and restore the skin barrier
  • Topical corticosteroids
  • Topical/oral antibiotics for secondary infection
  • Phototherapy/photochemotherapy
  • Calcineurin inhibitors 
  • Systemic immunosuppressants e.g. azathioprine

Complications

• Disseminated secondary eczema
• Lichenification
• Secondary bacterial infection