Novel compounds carrying ligands capable of binding to counter receptors on relevant target cells are disclosed. The compounds possess a number of advantageous features, rendering them very suitable for a wide range of applications, including use as detection systems, detection of relevant target cells as well as a number of other methods. In particular, novel MHC complexes comprising one or more MHC molecules are disclosed. The affinity and specificity of the MHC-peptide complexes are surprisingly high. The possibility of presenting to the target cells a plurality of MHC-peptide complexes makes the MHC complexes according to the present invention an extremely powerful tool e.g. in the field of therapy and diagnosis. The invention generally relates to the field of therapy, including therapeutic methods and therapeutic compositions. Also comprised by the present invention is the sample-mounted use of MHC complexes and MHC multimers.