Hypersensitivity is an immunological term that some pathologists have recently adopted as a histological diagnosis and some practitioners have started using clinically.

Immunologically speaking, hypersensitivity refers to the phenomenon in which certain predisposed individuals respond clinically to the antigens that the majority of individuals do not respond to. As such, immunological hypersensitivity is divided into four types.

Type I or anaphylaxis, is mediated by antigen, IgE antibody, and mast cells to produce hives and symptoms in other end organs, especially the respiratory tract. Acute allergic urticaria is the classic example.

Type II hypersensitivity reaction, also known as cytotoxic reaction , involves antigen–antibody interaction on the surface of cells such as red cells and platelets, resulting in anemia and thrombocytopenia. Autoimmune bullous disorders may be viewed as an example of type II reaction.

Type III hypersensitivity reaction, or immune complex disease spectrum , results from the deposition of antigen–antibody complexes on the endothelial cell surfaces of blood vessels or kidney, causing activation of complement, recruitment of inflammatory cells, and tissue destruction. Examples include many types of vasculitis and nephritis, including cutaneous leukocytoclastic vasculitis.