CURRENT VIEWS ON THE ALLOREACTIVE RESPONSE

Many new data have been obtained in the last 4 years about the structu re and the functions of Major Histocompatibility Complex (MHC) molecul es. Structural data are summarized and the functions of MHC molecules in the presentation of peptides to T cells is described. The high sele ctivity of peptides binding to MHC molecules is explained. The negativ e and positive selection of T cells in the thymus is described and the notion of repertoire introduced. The consequences of these new data o n the understanding of H2 restriction and alloreactivity are explained . The 4 potential types of alloreactivity are defined : 1) House keepi ng gene (HKG) peptides bound to allogenic MHC molecules, 2) allogenic peptides (derived from allogenic MHC molecules) bound to allogenic MHC molecules, 3) empty allogenic MHC molecules, 4) allogenic peptides bo und to autologous MHC molecules. In fact, the allogenic response is mo stly directed toward HKG peptides bound to the allogenic MHC molecules of the graft cells (type 1). The potential role of type 4 alloreactiv ity in rejection is discussed.