Transferrin receptor protein 1 (TfR1), also known as Cluster of Differentiation 71 (CD71), is a protein that in humans is encoded by the TFRCgene. TfR1 is required for iron import from transferrin into cells by endocytosis.
TfR1 is a transmembrane glycoprotein composed of two disulfide-linked monomers joined by two disulfide bonds. Each monomer binds one holo-transferrin molecule creating an iron-Tf-TfR complex which enters the cell by endocytosis.
TfR1 as a potential new target in cases of human leukemia & lymphoma. InatherYs, in Évry, France, developed a candidate drug, INA01 antibody (anti-CD71) that showed efficacy in pre-clinical studies in the therapy of two incurable orphan oncohematological diseases: the adult T cell leukemia (ATLL) caused by HTLV-1 and the Mantle cell lymphoma (MCL).
TfR1 expressed on the endothelial cells of the blood-brain barrier is used also in preclinical research to allow the delivery of large molecules including antibodies into the brain. Some of the TfR1 targeting antibodies have been shown to cross the blood-brain barrier, without interfering with the uptake of iron. Amongst those are the mouse anti rat-TfR antibody OX26 and the rat anti mouse-TfR antibody 8D3. The affinity of the antibody-TfR interaction seems to be important determining the success of transcytotic transport over endothelial cells of the BBB. Monovalent TfR interaction favors BBB transport due to altered intracellular sorting pathways. Avidity effects of bivalent interactions redirecting transport to the lysosome. Also, reducing TfR binding affinity directly promote dissociation from the TfR which increase brain parenchymal exposure of the TfR binding antibody .
 Pardridge, Buciak, and Friden, “Selective Transport of an Anti-Transferrin Receptor Antibody through the Blood-Brain Barrier in Vivo.”
^Pardridge WM, Buciak JL, Friden PM (October 1991). "Selective transport of an anti-transferrin receptor antibody through the blood-brain barrier in vivo". The Journal of Pharmacology and Experimental Therapeutics. 259 (1): 66–70. PMID1920136.
^Lee HJ, Engelhardt B, Lesley J, Bickel U, Pardridge WM (March 2000). "Targeting rat anti-mouse transferrin receptor monoclonal antibodies through blood-brain barrier in mouse". The Journal of Pharmacology and Experimental Therapeutics. 292 (3): 1048–52. PMID10688622.