Figure 2.

Schematic representation depicting processing of antigens presented by class II MHC molecules. 1) Extracellular and integral membrane proteins are internalized into endosomes via endocytosis. 2) Lysozomes fuse with endosomes. 3) Proteolytic degradation of endocytosed proteins occurs in the endolysozomal vesicle, resulting in the generation of antigenic peptides. 4) A specialized subcellualr organelle containing the class II MHC molecules, invariant chain, and DM fuses with the endolysozomal vesicle. This results in proteolytic degradation of invariant chain to CLIP. DM then catalyzes removal of clip, and the empty class II MHC molecules then bind antigenic peptides. 5) Class II MHC-antigenic peptide complexes are then exported to the cell surface, for presentation of antigenic peptide to CD4⁺ T cells.