As a result, class II MHC molecules cannot bind and present peptides they encounter in the ER,
leaving such peptides to associate with class I molecules.
The Ii also promotes folding and assembly of class II molecules and directs newly formed class II molecules to the late endosomes and lysosomes where internalized proteins have been proteolytically degraded into peptides.
Unlike vesicles containing proteins destined for secretion or for the cell surface, vesicles containing class II MHC molecules emerge from the Golgi complex and are targeted to late endosomes and lysosomes.
During their passage toward the cell surface, the exocytic vesicles transporting class II molecules out of the ER meet and fuse with the endocytic vesicles containing internalized and processed antigens