Types I, II, III, and IV Hypersensitivity

Abstract

Hypersensitivity refers to an increased reactivity or increased sensitivity by the animal body to an antigen to which it has been previously exposed. The term is often used as a synonym for allergy which describes a state of altered reactivity to an antigen. Hypersensitivity has been divided into categories based upon whether it can be passively transferred by antibodies or by specifically immune lymphoid cells. The most widely adopted current classification is that of Coombs and Gell that designates immunoglobulin-mediated (immediate) hypersensitivity reactions as types I, II and III and lymphoid cell-mediated (delayed-type) hypersensitivity/cell-mediated immunity as a type IV reaction. Hypersensitivity generally represents the “dark side” signifying the undesirable aspects of an immune reaction whereas the term immunity implies a desirable effect.