Which chains make up the peptide binding cleft in MHC class II molecules?

The correct response identifies the α1 and β1 chains as the components of the peptide binding cleft in MHC class II molecules. This cleft is critical for the function of MHC class II, which is to present processed peptides (derived from extracellular proteins) to CD4+ T cells.

In MHC class II molecules, the α1 and β1 regions come together to form a groove that specifically binds to these peptides. The structural arrangement allows for the appropriate presentation of a wide variety of peptide sequences, facilitating T cell recognition and activation. The precise binding occurs due to specific interactions between the amino acid residues of the peptide and pockets within the binding cleft formed by these two chains.

The other combinations presented do not form the peptide binding cleft: while the α2 and β2 chains contribute to the overall structure of the MHC class II family of proteins, they do not participate in forming the region where peptide binding occurs. CD4 is a co-receptor that assists in T cell activation but does not form part of the MHC molecule itself, distinguishing it from the structural components involved in peptide binding.