HLA class II histocompatibility antigen, DP alpha 1 chain (HLA-DPA1)

Binds peptides derived from antigens that get 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 get the endocytic path, where they're processed by lysosomal proteases and other hydrolases. Exogenous antigens that have been endocytosed by the APC are thus readily available for presentation via MHC II molecules, and that is why this antigen presentation pathway is usually referred to as exogenous.