“…The reasons for the discrepancy between MHC class I and MHC class II tetramers are still unclear. It is probably the convergence of many parameters: 1) low frequency of CD4 ϩ T cells (11,12,16,17), 2) low affinity of MHC-TCR interactions, 3) difficulties in the preparation of soluble class II-peptide complexes, 4) lower affinity of MHC class II-binding peptides as compared with class I-binding peptides, 5) sliding of the peptide within the groove (28,40,41), 6) importance of the flanking residues for binding and recognition, 7) nonequivalent functions of CD4 and CD8 accessory molecules, and 8) need to have active metabolic processes for staining (38). More importantly, it points out the fundamental differences between CD4 ϩ and CD8 ϩ T cells in term of physiology.…”