Recognizes proteins of 33, 35 and 41kDa, which are identified as various isoforms of CD74. Its epitope is localized in the extracellular domain of CD74. CD74 is a type II transmembrane protein which binds to the peptide binding groove of newly synthesized MHC class II alpha/beta heterodimers and prevents their premature association with endogenous polypeptides. The CD74 molecule plays a critical role in the presentation of peptides, by the MHC class II antigens, to CD4 positive lymphocytes. CD74 is expressed on MHC class II positive cells including B cells, a subset of activated T cells, monocytes, and dendritic cells and by various types of carcinomas. CD74 is expressed primarily by antigen presenting cells, such as B-lymphocytes (from before the pre-B cell stage to before the plasma cell stage), macrophages, and monocytes, and many epithelial cells. Anti-CD74 stains predominantly germinal center lymphocytes and B-cell lymphomas, but rarely T-cell lymphomas. Anti-CD74 has been shown to be useful in differentiating atypical fibroxanthoma (-) from malignant fibrous histiocytoma (+).