The invention refers to a synthetic peptide comprising an amino acid sequence of seven residues in length, preferably flanked by 2 cysteine residues at both ends, which is capable of specifically recognizing human CD154 and blocking CD40:CD154 interaction, thereby inhibiting the biological effects depending on such interaction. The peptide of the invention, which is preferably in a cyclic form, is suitable for use for diagnostic and therapeutic applications, especially for the diagnosis and therapy of tumor, inflammatory diseases and transplant rejection.
