MHC Class II Ultimers facilitate evaluation and characterization of CD4+ single antigen-specific T cells, enabling evaluation of the T cell immune response in a wide range of disease areas including, autoimmune disease, cancer and infectious diseases. Class II Ultimers are the next generation of MHC Class II multimers, with a peptide-linked planar design to improve the interaction between the MHC-peptide complexes and the T cell receptors, for consistent, reliable detection of CD4+ single antigen-specific T cells.