Hypersensitivity is best defined as:

Hypersensitivity is an exaggerated or inappropriate immune response to an antigen that causes tissue damage and clinical harm. Unlike a normal immune response, hypersensitivity leads to pathological inflammation and contributes to conditions such as allergies, autoimmune diseases, and transplant rejection. It is distinct from immunodeficiency, where the immune response is inadequate, and from autoimmune tolerance, where harmful self-reactive responses are suppressed. Understanding hypersensitivity is crucial for identifying and managing immune-mediated tissue injury effectively.

Reference: Robbins Basic Pathology, Kumar et al., 10th Edition.