To aid in the identification of antigen-specific T cells, BioLegend is proud to launch Flex-T Major Histocompatibility Complex (MHC) tetramers, a novel technology licensed from Sanquin Blood Supply Foundation. The T-cell–mediated immune response is defined by the interaction between antigen-presenting cells and T cells, through the MHC and the T-cell receptor (TCR). MHC molecules present a peptide to antigen-specific T cells that recognize this peptide. Soluble, monomeric MHC molecules bind very weakly to the TCR. However, by making a tetramer using a fluorescently labeled streptavidin conjugate, the complex has greater avidity to the T cell and maintains more stable binding by interacting with several TCRs, making it useful for flow cytometry detection of antigen-specific T cells. Flex-T has the unique property of allowing the loading of peptides of interest into the binding site of the MHC groove, by using UV light–labile, exchangeable peptides.