Allergy is the term given to a reaction by a small number of people to a substance (known as the allergen) which is harmless to those who are not allergic to it. Only small quantities of allergen are necessary to induce the reaction. Contact allergy occurs predominantly from the allergen on the skin rather than from internal sources or food. The first contact does not result in allergy; often the person has been able to touch the material for many years without adverse reaction.

Allergic contact dermatitis is an itchy skin condition caused by an allergic reaction to material (the allergen) in contact with the skin. It arises some hours after contact with the responsible material, and settles down over some days providing the skin is no longer in contact with it. In severe cases contact allergic dermatitis may be followed by generalized auto-eczematization (id reaction). Ingestion of a contact allergen is usually safe, but rarely may lead to baboon syndrome or generalized systemic contact dermatitis.

Contact dermatitis should be distinguished from contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours. The allergic reaction to latex is the best known example of allergic contact urticaria.

Allergic contact dermatitis is also distinct from irritant contact dermatitis, in which a similar skin condition is caused by excessive contact with irritants. Irritants include water, soaps, detergents, solvents, acids, alkalis, and friction. Irritant contact dermatitis may affect anyone, providing they have had enough exposure to the irritant, but those with atopic dermatitis are particularly sensitive. Most cases of hand dermatitis are due to contact with irritants.

Acute allergic contact dermatitis is characterized by pruritic papules and vesicles on an erythematous base. Lichenified pruritic plaques may indicate a chronic form of the condition.

Individuals with allergic contact dermatitis typically develop the condition within a few days of exposure, in areas that were exposed directly to the allergen. Certain allergens (eg, neomycin), however, penetrate intact skin poorly; in such cases, the onset of dermatitis may be delayed for up to a week following exposure. Individuals may develop widespread dermatitis from topical medications applied to leg ulcers or from cross-reacting systemic medications administered intravenously. Intraoral metal contact allergy may result in mucositis that mimics lichen planus, which has an association with intraoral squamous cell carcinoma.