text: T cells play an important role in the adaptive immune system. They are capable of orchestrating, regulating and coordinating complex immune responses. A wide array of clinically relevant aspects are associated with the function or malfunction of T-cells: Autoimmune diseases, control of viral or bacterial pathogens, development of cancer or graft versus host responses.
  Over the past years, various methods (ELISpot Assay, intracellular cytokine staining, secretion assay) have been developed for the identification of T cells, but only major histocompatibility complex (MHC) procedures allow identification and purification of antigen-specific T cells independent of their functional status.
  In principle, MHC procedures are using the T cell receptor (TCR) ligand, which is the MHC-peptide complex, as a staining probe. The MHC interacts with the TCR, which in turn is expressed on the T cells. Because TCR-MHC interactions have only a very weak affinity towards each other, monomeric MHC-epitope complexes cannot provide stable binding. This problem can be solved by using multimerized MHC-epitopes, which increases the binding avidity and therefore allows stable binding. Fluorochromes conjugated to the MHC-multimers then can be used for identification of T cells by flow cytometry.
  Nowadays, MHC molecules can be produced recombinantly together with the antigenic peptides which are known for a fast-growing number of diseases.