Vol. 10, Issue 12, Part G (2025)

Exaggerated or inappropriate immune responses, also known as hypersensitivity reactions, result in tissue injury and contribute to a wide spectrum of allergic, autoimmune, and inflammatory diseases. These reactions arise from dysregulated interactions among immune cells, antibodies, and inflammatory mediators following prior sensitization to specific antigens. Gell and Coombs classified hypersensitivity reactions into four types that is Type I (IgE-mediated immediate hypersensitivity), Type II (antibody-mediated cytotoxic reactions), Type III (immune complex-mediated reactions), and Type IV (T-cell-mediated delayed hypersensitivity). The immunological mechanisms underlying each type, including the roles of immunoglobulins, complement activation, immune complexes, cytokine networks, and effector immune cells are discussed. Furthermore, Type II and Type IV hypersensitivity reactions are subdivided to highlight the functional diversity of immune-mediated tissue injury. Understanding the mechanistic basis of hypersensitivity reactions is essential for accurate diagnosis, effective management, and the development of targeted therapeutic strategies.