 The antigen binding groove, where the antigen or peptide binds, is made up of two α-helixes walls and β-sheet.Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24 amino acid residues long.These molecules are constitutively expressed in professional, immune antigen-presenting cells, but may also be induced on other cells by interferon γ.