Within hours after the skin of a sensitized person has come into contact with a particular allergen, red macules or patches develop that usually evolve rapidly, sometimes through an intermediate stage of urticarial papules or plaques, into vesicles that may become bullae. Sometimes, however, allergic contact dermatitis may present itself only as reddish papules or papulovesicles, but more often as tense vesicles as well. Episodically, depending on the patient’s degree of hypersensitivity, bullae may come into being. If the offending agent is removed, lesions of allergic contact dermatitis, no matter how widespread or severe, resolve, even without therapy, in a matter of weeks at most. If, however, the allergen continues to come into contact with the skin, the process may persist for years. In that latter situation, annoying pruritus leads to persistent rubbing, which causes lichen simplex chronicus to be imposed on the effects of longstanding allergic contact dermatitis.
The same general principles that apply to allergic contact dermatitis apply equally to irritant contact dermatitis, the earliest stage being red macules or patches that very soon are surmounted by vesicles or bullae, or both. In the case of irritant-induced blisters, however, the roof of the lesions often has a gray cast, a consequence of the epidermis having become necrotic secondary to the effects of the irritant. Although both allergic contact dermatitis and irritant contact dermatitis result from direct contact of the skin with an offending agent, the two mechanisms are completely different; allergic contact dermatitis results from immunologic mechanisms, whereas irritant contact dermatitis does not.