Contact dermatitis is an inflammation of the skin that can be acute or chronic that manifests as eczematous dermatitis due to exposure to substances in the environment.

Allergic contact dermatitis is an immunologic cell-mediated skin reaction to exposure to antigenic substances.
The lesions initially appear on the cutaneous site of principal exposure then may spread to other more distant sites due to contact or autosensitization. Lesions are typically asymmetrical and unilateral.
Specific signs and symptoms will depend on the duration, location, degree of sensitivity and concentration of allergens. The patch test shows reaction to allergen.
Irritant contact dermatitis is a non-immunologic skin reaction to skin irritants.
It is often localized to areas of thin skin eg eyelids, intertriginous areas.