Characteristics of irritant and allergic contact dermatitis

	Irritant contact dermatitis	Allergic contact dermatitis
Etiology	Irritant substances	Allergens
Mechanisms	Nonimmunologic	Delayed-type hypersensitivity Antigen-specific T cells
Sites	Localized to the contact areas	At the site of the contact May be widespread
Symptoms	Burning, stinging, and pain Pruritus may be present	Pruritus
Onset	May appear after first contact Usually minutes to hours (strong irritant) or 24 hours to 1 week (weak irritant)	Sensitization phase before onset Usually 24 to 72 hours after exposure
Decrescendo phenomenon	Yes	No
Clinical manifestation
Acute stage	Erythema, edema, vesicles, and bullae Necrosis and ulceration in severe cases	Erythema, edema, vesicles
Chronic stage	Fissuring, hyperkeratosis, and scaling	Fissuring, hyperkeratosis, and scaling
Histopathology	Mild spongiosis, necrosis of epidermal keratinocyte	Spongiosis
Patch testing	Negative	Positive

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