In one aspect, the disclosure relates to major histocompatibility complex (MHC) molecules comprising a ligand in the peptide binding groove of the MHC molecule, whereby the ligand comprises an azobenzene (Abc), and at least two amino acid residues separated by the azo-group of the Abc, and wherein the amino acid residues are positioned to interact with the peptide binding groove of the MHC molecule. The disclosure also relates, among others, to means and methods for producing and using such MHC molecules, and the ligands therefor.