Hypersensitivity is defined as an exaggerated immune response that leads to tissue damage and clinical symptoms rather than protection. It involves overactivation of immune cells or antibodies against harmless antigens or self-antigens, resulting in inflammation and injury. Unlike a normal immune response, hypersensitivity causes harmful effects such as allergic reactions, autoimmune diseases, or chronic inflammation. Recognizing this helps in diagnosing and managing conditions where immune modulation is necessary to prevent damage.

Reference: Robbins Basic Pathology, Kumar, Abbas, Aster, 10th Edition.