What is an “id” reaction, and what does it have to do with Sigmund Freud?

An id reaction, also known as autosensitization dermatitis, is an immunologically mediated cutaneous inflammation in the absence of locally viable organisms or other locally inciting cause. It presents suddenly as an acute, monomorphous, papulovesicular dermatitis distant to an area of primary dermatitis. It usually erupts symmetrically on the hands, forearms, flexor aspects of the arms, extensor aspects of the arms and thighs, and, less commonly, on the face and trunk. Juicy papules often coalesce into small plaques with associated red macules or wheals. Sigmund Freud did not discover id reactions. In dermatologic usage, “id” derives from a Greek suffix for a father–son relationship or resemblance. Freud’s id derives from a third-person Latin pronoun.