Epidemiology
Etiology
Pathophysiology
Allergic contact dermatitis is an example of a type IV hypersensitivity reaction.
- Sensitization Phase (1st exposure, 10-14 days)
- Hapten (nickel, urushiol) binds carrier protein → complete antigen
- Langerhans cells capture antigen, migrate to lymph nodes
- Present via MHC II to CD4+ T cells → Th1 cell activation & memory formation
- No symptoms yet
- Elicitation Phase (Re-exposure, 24-72 hrs)
- Memory Th1 cells recognize antigen
- Release IFN-γ, IL-2, TNF-α
- Recruit macrophages & CD8+ T cells → keratinocyte damage
- Result: pruritic, erythematous, vesicular rash with spongiosis
- Key Point: No antibodies or mast cells (vs Type I)
Clinical features
- Intensely pruritic erythematous papules
- Vesicles with serous oozing in more severe cases
Tip
Contact dermatitis due to poison oak, poison ivy, or poison sumac is the most likely cause in a patient presenting with erythematous, pruritic, and burning skin lesions in a linear pattern that appear 24 hours after a camping trip.
Diagnostics
