CD74 plays a critical role in MHC class II antigen processing by stabilizing peptide-free class II alpha/beta heterodimers in a complex soon after their synthesis and directing transport of the complex from the endoplasmic reticulum to the endosomal/lysosomal system where the antigen processing and binding of antigenic peptides to MHC class II takes place. CD74 also serves as cell surface receptor for the cytokine MIF. [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/P04233].