 The antigens presented by class II peptides are derived from extracellular proteins (not cytosolic as in MHC class I).Loading of a MHC class II molecule occurs by phagocytosis; extracellular proteins are endocytosed, digested in lysosomes, and the resulting epitopic peptide fragments are loaded onto MHC class II molecules prior to their migration to the cell surface.In humans, the MHC class II protein complex is encoded by the human leukocyte antigen gene complex (HLA).