Allergic Contact Dermatitis (ACD)
Allergic Contact Dermatitis is a Type IV (delayed-type) hypersensitivity reaction occurring when the skin comes into contact with a specific allergen to which the patient has been previously sensitized.
Clinical Features
- Presentation: Intense pruritus, erythema, papules, and vesicles occurring 24–48 hours after exposure.
- Distribution: Often limited to the area of contact (e.g., watch strap, jewelry, or specific plant exposure).
- Chronic Phase: May lead to lichenification and scaling if the allergen is not identified and removed.
Common Allergens
| Allergen | Source |
|---|---|
| Nickel | Jewelry, belt buckles, eyeglass frames. |
| Rhus (Urushiol) | Poison ivy, poison oak, poison sumac. |
| Preservatives | Formaldehyde, parabens in cosmetics/creams. |
| Fragrances | Perfumes, soaps, lotions. |
High-Yield Exam Pearl:
- Poison Ivy (Rhus dermatitis): Characterized by “linear streaks” of vesicles at the site of contact. Note that the fluid in the vesicles does not contain allergen and cannot spread the rash to others.
- Patch Testing: The definitive diagnostic tool. It is different from skin prick testing, which is used for IgE-mediated (Type I) immediate hypersensitivities (like food or pollen allergies).