LN-2 is directed against the CD74 cluster, established during the IVth Leukocyte Typing Workshops. CD74 comprises four species of proteins (MW 41/35/33 kDa), all coded by a single gene, consisting of nine exons. 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. In tissue sections anti CD74 show a binding pattern very similar to that of anti-HLA-DR. It binds to the peptide binding groove of newly synthesized MHC class II alpha/beta heterodimers and prevents their premature association with endogenous polypeptides. Positive control: Daudi or Raji cells. Tonsil or lymp node.