The present invention provides compositions and methods of inhibiting or inducing activation of T cells in a patient. The methods comprise administering a therapeutically effective dose of pharmaceutical compositions comprising a pharmaceutically acceptable carrier and peptides of between about 4 and about 20 residues, that bind antigen binding sites on MHC molecules encoded by substantially all alleles of a DR locus. These peptides are referred to as pan DR binding peptides. The pan DR binding peptides can be used to inhibit immune responses associated with immunopathologies, such as autoimmunity, allograft rejection and allergic responses. The peptides can also be used in combination with CTL peptides to enhance a CTL response.