CD74 (invariant chain, Ii) is a non-polymorphic type II transmembrane protein that primarily serves as a chaperone for MHC class II molecules, guiding their assembly, transport, and peptide loading. It is also a cell-surface receptor for macrophage migration inhibitory factor (MIF), and MIF-CD74 interaction activates pro-inflammatory and survival pathways (e.g., NF-κB, MAPK).