Most T lymphocytes express a clonal and highly specific antigen receptor (TCR) on the cell surface. TCR (T cell receptor) binds to the MHC-peptide complex and recognizes the antigenic peptide presented in the context of MHC molecules. Because TCR has a low avidity and fast off-rates for MHC-peptide complexes, recombinant soluble monomeric MHC-peptide complexes have not been used to detect antigen-specific T cells. Altman et al. (Science 1996, 274: 94-96) have introduced MHC-peptide tetrameric complexes (so-called MHC tetramers) for detection of antigen-specific T cells. MBL MHC tetramer technology is based on the ability of MHC-peptide complexes to recognize the antigen-specific T cells at a single cell level. This breakthrough technology enables researchers to precisely measure targeted T-cell responses in infectious diseases, cancer, and autoimmune diseases.