HLA class II histocompatibility antigen, DRB1-5 beta chain (DR-5) is a heterodimer of an alpha and a beta subunit; also referred as MHC class II molecule. DR-5 binds peptides derived from antigens that access the endocytic route of antigen presenting cells (APC) and presents them on the cell surface for recognition by the CD4 T-cells. The peptide binding cleft accommodates peptides of 10-30 residues. The peptides presented by MHC class II molecules are generated mostly by degradation of proteins that access the endocytic route; where they are processed by lysosomal proteases and other hydrolases.