Model of HLA class II. The HLA class II molecule is composed of 2 polypeptide chains, α and β. The heterodimer forms a peptide-binding groove. The α1 domain, peptide, and β1 domain is the trimolecular complex recognized by the T-cell receptor (TCR). The molecular model was generated and kindly provided by Peter Hjelmström (Department of Medicine, Karolinska Institute, Stockholm, Sweden).