Abstract: This invention concerns an oligomeric receptor-ligand pair member in general and an oligomeric MHC-peptide complex in particular and a method of labeling, detecting and separating mammalian T cells according to the specificity of their antigen receptor by use of the oligomer. The invention further concerns a method of targeting the oligomeric receptor-ligand pair member complexes to target molecules of the surface of a target cell in order to present antigens on the target cell. The invention further concerns related pharmaceutical and diagnostic compositions and processes.