CD8+ T cells are essential components of the immune system. They function through TCR recognition of antigens (peptide epitopes) presented by the class I MHC (HLA in humans) molecules. Thus, TCR recognition of cognate MHC/peptide complex can be utilized for the detection of antigen-specific CD8+ T cells. Due to the intrinsic low affinity of MHC/TCR interaction, the detection of antigen specific T cells requires enhanced binding avidity by multimerization of MHC/peptide monomers, ensuring the interaction of multiple MHC/TCR simultaneously. This can be accomplished by using streptavidin to tetramerize biotinylated MHC class I complexes.In the absence of proper peptide binding, the HLA class I complex will quickly disassociate. Flex-T? UVX (Ultraviolet Exchange) monomer is designed as a biotinylated α-chain, associated with the β2-microglobulin chain, and stabilized by a UV-labile peptide. The C-terminus of the α-chain is site-specifically biotinylated by the enzyme BirA. When the complex (monomer) is irradiated with UV in the presence of a new peptide with proper affinity, it will result in a net peptide exchange. The new peptide replaces the pre-loaded UV-labile peptide and forms a new MHC monomer. The new monomers can be then tetramerized and used for labeling antigen-specific T cells or other studies.