FIGURE 23-26 Class II MHC pathway of antigen processing and presentation. Step 1: Particulate antigens are acquired by phagocytosis and nonparticulate antigens by pinocytosis or endocytosis. Step 2:Exposure of antigen to the acidic and reducing environment of endosomes and lysosomes prepares the antigen for proteolysis. Step 3:The antigen is broken down by various proteases in endosomal and lysosomal compartments. Step 4: Class II MHC molecules, assembled in the ER from their subunits, are delivered to endosomal and lysosomal compartments by means of signals contained in the associated invariant (Ii) chain. This delivery targets late endosomes, lysosomes, and early endosomes, ensuring that class II MHC molecules are exposed to the products of proteolytic breakdown of antigen along the entire endocytic pathway. Step 5: Peptide loading is accomplished with the assistance of DM, a class II MHC–like chaperone protein. Step 6: Peptide-loaded class II MHC molecules are displayed at the cell surface. See text for details.