Class II HLA molecules are involved in the antigen presentation pathway. Initially, exogenous antigens are taken up by APCs through phagocytosis or endocytosis. These antigens are then degraded into peptides within endosomal/lysosomal compartments. Class II HLA molecules are synthesized in the endoplasmic reticulum and transported to these compartments, where they bind to the antigenic peptides. The HLA-peptide complex is then transported to the cell surface for recognition by CD4+ T cells.
