Eczematous reaction from skin contact with chemical irritants (ICD) or allergens triggering type IV hypersensitivity (ACD).
Also known as: contact dermatitis, allergic contact dermatitis, irritant contact dermatitis, ACD, ICD, poison ivy
Overview
Inflammatory dermatitis caused by direct contact with an exogenous agent. Two main forms: irritant contact dermatitis (ICD, non-immunologic toxic injury) and allergic contact dermatitis (ACD, T-cell mediated type IV delayed hypersensitivity).
Epidemiology
ICD accounts for ~80% of all contact dermatitis; most common occupational skin disease (hairdressers, healthcare, cleaning, construction, food handling). ACD prevalence ~20%; nickel is the most common allergen worldwide.
Try two board-style Allergic and Irritant Contact Dermatitis questions
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Question 1DermatologyMedium
A 45-year-old female develops red itchy linear streaks on her arm 2 days after gardening. The pattern follows where leaves brushed her skin. Which of the following is the most likely diagnosis?
AIrritant contact dermatitis
BStasis dermatitis
CAtopic dermatitis
DAllergic contact dermatitis
Reveal answer & full explanation
Correct answer: D — Allergic contact dermatitis
AIrritant contact dermatitis
BStasis dermatitis
CAtopic dermatitis
DAllergic contact dermatitis✓
Why Allergic contact dermatitis is correct
Allergic contact dermatitis is a type IV (delayed) hypersensitivity reaction, T-cell mediated, requiring prior sensitization
Onset occurs 24–72 hours after re-exposure to the allergen
The linear streak pattern following where leaves brushed the skin is characteristic of Toxicodendron (poison ivy/oak/sumac) contact — the urushiol oil causes this linear distribution
Treatment: avoid the allergen, topical mid-to-high potency corticosteroids, oral corticosteroids if severe
Patch testing identifies the specific causative allergen
Why the others are wrong
Irritant contact dermatitis — more common than allergic; non-immunologic (no prior sensitization required); caused by direct cellular damage from soap, detergent, or solvent; typically presents with more burning/stinging rather than pruritus and lacks discrete borders
Stasis dermatitis — affects the lower legs in the setting of venous insufficiency; not triggered by plant contact
Atopic dermatitis — chronic pruritic eczema in flexural distributions; not caused by plant contact and does not produce linear streaks
Additional high-yield points
Most common cause of allergic contact dermatitis in adults: nickel (found in jewelry, snaps, watches)
Distinguishing allergic from irritant contact dermatitis: allergic is typically more pruritic with discrete borders; irritant causes more burning/stinging
Question 2DermatologyMedium
A 34-year-old landscaper presents 2 days after clearing brush, reporting an intensely pruritic rash. Examination shows linear streaks of erythema, edema, and vesicles across both forearms, the neck, and the face, involving roughly 25% of the body surface area. The eruption is weeping in several areas. He has no fever and no signs of secondary infection. Which of the following is the most appropriate initial management?
AOral hydroxyzine for itch relief
BOral prednisone for 2 to 3 weeks
COral cephalexin for 7 to 10 days
DOral methylprednisolone for 6 days
Reveal answer & full explanation
Correct answer: B — Oral prednisone for 2 to 3 weeks
AOral hydroxyzine for itch relief
BOral prednisone for 2 to 3 weeks✓
COral cephalexin for 7 to 10 days
DOral methylprednisolone for 6 days
Why Oral prednisone for 2 to 3 weeks is correct
This is severe, widespread allergic contact dermatitis (urushiol-induced poison ivy), a type IV delayed hypersensitivity reaction, with facial involvement and ~25% BSA, which is beyond what topical therapy can control.
Guideline-supported management of severe or widespread ACD is a systemic corticosteroid: oral prednisone 0.5-1 mg/kg/day tapered over 14-21 days. Tapers shorter than 14 days cause rebound dermatitis, so the longer course is required.
Why the others are wrong
Oral methylprednisolone for 6 days: a real systemic steroid, but a course under 14 days is the classic poison ivy mistake; rebound flare is common, so the dose pack is inadequate for this severity.
Oral cephalexin for 7 to 10 days: an antibiotic, indicated only with secondary bacterial infection. This patient has no purulence, fever, or honey-crusting, so it does not treat the underlying dermatitis.
Oral hydroxyzine for itch relief: a sedating antihistamine that may aid sleep, but it has minimal direct antipruritic effect in contact dermatitis and does not address the widespread inflammatory reaction.
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Antibiotic only if secondary bacterial infection — cephalexin, dicloxacillin
Topical antipruritic: pramoxine, menthol
Refer to occupational medicine for workers' compensation documentation
Complications
Secondary bacterial infection (S. aureus, S. pyogenes)
Post-inflammatory hyperpigmentation, especially Fitzpatrick IV-VI
Chronic occupational hand dermatitis → job loss, disability
Erythroderma in widespread severe cases
Steroid-induced atrophy with prolonged high-potency topical use
PANCE pearls
Linear streaky vesicles after hiking = poison ivy / oak / sumac (urushiol). The fluid in vesicles does NOT spread the rash; new lesions reflect different exposure doses absorbed at different rates.
Eyelid dermatitis from nail polish (tosylamide formaldehyde resin) is classic — patient touches eyes with allergen-coated nails.
Nickel patch testing is positive in ~15-20% of women and ~5% of men; the dimethylglyoxime spot test identifies nickel-releasing metal jewelry.
Topical neomycin and bacitracin are common ACD culprits — avoid prophylactic 'triple antibiotic' on clean wounds; petrolatum is sufficient.
Oral prednisone for poison ivy MUST be tapered ≥14-21 days; shorter courses cause rebound.
References
AAD/ACDS 2020 — American Contact Dermatitis Society Core Allergen Series (Schalock et al., Dermatitis 2020)
NACDG — North American Contact Dermatitis Group Patch-Test Results Periodic Reports (DeKoven et al., Dermatitis)
AAD 2006 — Guidelines of Care for Contact Dermatitis (Bourke et al., J Am Acad Dermatol; updated reference standards)
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