What type of hypersensitivity reaction is allergic contact dermatitis?

Allergic contact dermatitis is a Type IV (delayed-type, T-cell/cell-mediated) hypersensitivity reaction, developing over 24-72 hours after re-exposure to a sensitizing allergen. This differs from irritant contact dermatitis, which is a non-immunologic reaction caused by direct chemical/physical irritation or friction, not an allergic mechanism at all.