Hypersensitivity

It is known that the adaptive immune response is a significant constituent of host defense against infections and hence essential for maintaining normal health. The immune responses in the form of either cell mediated or humoral mediated, sometimes elicited by antigens not associated with infectious agents, are inappropriate and this may lead to causation of serious disease. The inappropriate immune response, rather than providing exemption or safety to the affected animal, can produce severe and occasionally fatal results. These are known as “hypersensitivity reactions” and the diseases caused are known as hypersensitivity diseases. Coombs and Gell in 1963 defined four types of hypersensitivity based on immunologic mechanisms involved. The first three types I to III are immunoglobulin-dependent reactions involving a humoral B-lymphocyte-mediated response. Conversely, type IV is a T-cell-mediated hypersensitivity reaction. Overreaction to antigen is also described as “allergy,” the term originally coined by Von Pirquet early in the twentieth century. Allergy is defined as the altered reactivity of an animal following exposure to a foreign antigen and included both immunity and hypersensitivity. The term, however, is now restricted to refer only to the hypersensitivity, which may be associated with the development of the immune response to a foreign substance. The essential basis of these hypersensitivity reactions is briefly summarized here and then each will be described separately in greater detail.