The invention relates to a method of selecting HLA-DP4 ligand molecules comprising the following steps: (i) incubation of purified HLA-DP4 with a tracer having general formula (I) Z1X1X2X3X4X5X6X7X8X9Z2 (wherein Z1 and Z2, which may be identical or different, are equal to zero or each represent a peptide with between 1 and 100 amino acids; X6 represents an aromatic or hydrophobic amino acid or a cysteine; and X1 represents an aromatic or hydrophobic amino acid and/or X9 represents an aromatic or hydrophobic amino acid or C, D, Q, S, T or E; and X2,X3,X4,X5,X7 and X8 each represent an amino acid) in the presence of different concentrations of molecule(s) to be tested; (ii) separation of the different complexes formed; (iii) detection of HLA-DP4/peptide tracer complexes; and (iv) selection of ligand molecules having a binding capacity IC50