Allergic contact sensitisation

**Definition:** The presence of specific delayed type IV hypersensitivity of the immune system to a given substance without imputation of past or current disease. Such sensitisation is normally acquired by prior contact of the skin or mucous membranes with the substance or with one chemically closely related to it (cross-reactivity). Subsequent contact with the substance may provoke an allergic reaction. In certain circumstances such sensitisation may prevent an individual from taking up or continuing employment where exposure to the allergen cannot be avoided. Some individuals sensitized to a specific allergen may, however, never experience symptoms on contact with it.