What Is Hypersensitivity Reaction?
“Hypersensitivity” is an immunological term that has acquired a histological meaning by many pathologists. Predisposed individuals respond to antigens that a majority of individuals do not respond to. As such, hypersensitivity reaction (HR) is divided into four types: Type I (anaphylaxis), Type II (cytotoxic reaction), Type III (immune complex), and Type IV (cell mediated).
The histological term HR does not refer to any specific immunological process but instead to an inflammatory reaction of the dermis that does not conform to other known dermal disorders. The diagnosis is often made in the face of an infiltrate of lymphocytes and eosinophils, which may be seen in disorders with known causes such as reaction to arthropod bite, scabies, urticaria, pruritic urticarial papules and plaques of pregnancy (PUPPP), some drug eruption, and disorders of unknown etiology such as prurigo simplex. Because of the overlap in the histological findings among the above disorders, the diagnosis of HR may be made and is an invitation to search for a possible cause.