The MHC class II antigen processing and presentation pathway

This is a figure showing antigen processing and presentation via cells utilizing MHC class II. Initially, exogenous antigen is taken up and broken down by proteases (at acidic pH) into peptides in the endocytic compartment. MHC class II alpha-beta heterodimers are assembled in the ER with the invariant chain (Ii) polypeptide. These complexes are targeted from the Golgi complex to the endocytic compartment by Ii, where they encounter peptides generated from exogenous antigen. Ii is degraded to the CLIP peptide by proteases at low pH, and CLIP exchange is facilitated by acidic conditions and catalyzed by HLA-DM. MHC II molecules that have stably bound new peptides are then exported to the cell surface for presentation to CD4+ T cells.