HLA class II histocompatibility antigen, DRB1 beta chain (HLA-DRB1)

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 peptides presented by MHC class II molecules are generated mostly by degradation of proteins which access the endocytic path; where they are processed by lysosomal proteases and other hydrolases.

Exogenous antigens which have been endocytosed from the APC are thus readily available for presentation via MHC II molecules; and for this reason this antigen presentation pathway is usually known as exogenous. As membrane proteins on their way to degradation in lysosomes as part of their usual turn-over are also contained in the endosomal/lysosomal pockets; exogenous antigens must compete with those derived from endogenous components.