Equine immunology 2: immunopharmacology--biochemical basis of hypersensitivity.

In general, 4 types of hypersensitivity reactions can be defined according to their immunological basis and clinical appearance. The differing mechanisms of these responses are described with particular reference to chemical mediators which through their pharmacological actions contribute to the clinical manifestations of hypersensitivity. Chemical mediators may exert their influence locally or systemically through their action on effector, tissues or organs and in addition, may be involved in the recruitment of cells of specific type to the site of the reaction. The possible role of these mediators in hypersensitivity in the equine species is discussed to provide a background for a subsequent paper which will be concerned with the therapy of hypersensitivity conditions.