The antibody is directed against the transferrin receptor (CD71), the 95 kDa, type II membrane glycoprotein. CD71is expressed on activated, but not resting lymphocytes, monocytes, macrophages, most dividing cells and on brain endothelium, and is upregulated during response to antigens and/or mitogens. The transferrin receptor binds Fe(Apo)-transferrin and via endocytosis and delivers iron required by the growing cell for cellular metabolism.