Immunological basis and classification of allergic reactions.

The task of the immune system consists of destroying foreign structures in order to prevent the invasion of living organisms by pathogens. This destru ction often provokes adverse effects defined as hypersensitivity reactions. The classification of hypersensitivity reactions proposed by Gell and Coom bs is based on effector mechanisms. Although this framework remains largely used recent work demonstrated the occurrence of profound similarities betw een different classes of hypersensitivity reactions. Indeed, all these reac tions involve the activation of adhesion mechanisms intended to allow the l ocal recruitment of blood leukocytes and cytokine production. The concept o f a separation between two different modes of immune response induced by tw o different T lymphocyte subpopulations (TH1 and TH2) proved useful in unde rstanding hypersensitivity reactions, although it represents only an approx imation. Recent results were an incentive to develop new diagnostic and the rapeutic strategies whose usefulness will be evaluated in site near future. (C) 1999 Elsevier, Paris.