The present invention relates to the use of therapeutic compounds in the modification of T-cell, T-cell-antigen presenting cell (APC) interactions and the interactions between pathogenic organisms and immunocompetent cells of a host. In particular it relates to the use of these compounds in the modulation of the interaction between Notch proteins and their ligands and to the use of such compounds in the therapy of conditions such as graft rejection, autoimmunity, allergy, and asthma and infectious diseases.