It recognizes a protein of ~35 kDa，identified as 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. 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 a typical fibroxanthoma (-) from malignant fibrous histiocytoma (+).
