The OKT9 monoclonal antibody specifically binds to CD71 which is also known as the transferrin receptor (TFR or Transferrin R). CD71 (Transferrin R) is a type II transmembrane glycoprotein that is encoded by TFRC (transferrin receptor) which belongs to the Transferrin receptor family. It is expressed on cells as a disulfide-linked homodimer comprised of ~95 kDa monomers. CD71 is expressed on early thymocytes as well as activated lymphocytes, monocytes, macrophages, brain endothelium, and most proliferating cells. The transferrin receptor is also present on early erythroid cells but is lost as reticulocytes differentiate into mature erythrocytes. CD71 is lowly expressed on normal resting lymphocytes and is upregulated during lymphocyte responses to antigens or mitogens. The transferrin receptor is essential for iron transport into proliferating cells. Through an endocytic pathway, the transferrin receptor mediates cellular iron uptake by binding and internalizing iron that is bound to transferrin. After releasing iron within the low pH endosomal environment, transferrin and its receptor can be recycled to the cell surface.