CD74, also known as the MHC class II invariant chain (Ii), is a non-polymorphic type II transmembrane glycoprotein that plays a crucial role in the immune system. Its primary function is to assist in the presentation of exogenous antigen peptides through MHC II molecules, initiating a specific humoral immune response. It acts as a chaperone for MHC class II molecules, helping in the folding, transport, and peptide loading of these molecules. Beyond its role in antigen presentation, CD74 is also a receptor for macrophage migration inhibitory factor (MIF), a cytokine involved in various inflammatory conditions, autoimmune diseases, and cancer. MIF binding to CD74 leads to signaling events such as ERK activation, cell proliferation, inhibition of apoptosis, and prostaglandin E2 production. CD74 is frequently observed in tumor cells of different origins, suggesting its implication in tumorigenesis and potential as a therapeutic target. Its expression is associated with poor prognosis, possibly due to suppressive effects on the immune system.