The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 32 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. CD74, also known as HLA class2 histocompatibility antigen gamma chain and HLA-DR antigens-associated invariant chain, is a polypeptide involved in the formation and transport of MHC class2 protein. CD74 is expressed by B cells, macrophages, and Reed-Sternberg cells. When MHC class 2 protein was in the rough ER, its peptide-binding cleft was blocked by CD74 to prevent it from interacting with the endogenous peptides. CD74 also serves to facilitate MHC class2's export from ER.