Agents for blocking T cell-mediated immune reactions are provided. Such agents are peptides, referred to as "CD28 peptide mimetics", of from 15 to 30 amino acids in length. The CD28 peptide mimetics comprise the hexapeptide motif `MYPPPY`, SEQ ID NO: 1, or a retro-inverso isomer thereof. The CD28 peptide mimetics further comprise flanking sequence at the amino and carboxyl terminus of the hexapeptide motif. Methods for treating subjects with T cell mediated autoimmune diseases or disorders are also provided. Such methods comprise administering one or more of the CD28 peptide mimetics to a subject with such a disease or disorder.