CD74, a non-polymorphic type II transmembrane glycoprotein. It traditionally functions as a chaperone for major histocompatibility complex class II (MHC II) molecules, regulating their trafficking and antigen loading in antigen-presenting cells. Beyond this role, CD74 can act as a cell surface signaling receptor when bound by its natural ligand, Macrophage Migration Inhibitory Factor (MIF). The CD74-MIF interaction triggers intracellular pathways (such as NF-κB and MAPK) that promote cell proliferation, survival, and the secretion of pro-inflammatory cytokines.