T-cell staining with peptide-major histocompatibility complex (MHC) multimers. (a) Specific cells can be labeled directly ex vivo. (b) The peptide-MHC multimer consists of a complex of four peptide-MHC-biotin monomers complexed with avidin and directly conjugated to fluorescein. Counterstaining for CD3 helps to ensure specificity of the staining, but can be replaced for any other parameter (eg secreted cytokines; see Fig. 5).