CD71 is a homodimeric type II transmembrane glycoprotein consisting of two disulfide-bonded 95 kDa chains. CD71 is the transferrin receptor (TfR). The CD71 binds iron saturated holo-transferrin on the cell surface and is internalized and transported to early sorting endosomes, where the complexed iron is rapidly separated from transferrin. Subsequently, the CD71-transferrin complex accumulates within recycling endosomes and then recycles to the plasma membrane. On the cell surface, iron-free apotransferrin has negligible affinity for CD71, and is released from the cell, ready to begin a new cycle. CD 71 is expressed on activated B and T cells, macrophages, reticulocytes, erythroid precursors, and malignant tissues. Particularly high expression of CD71 is reported in the reticulocytes and erythroid series, reflecting the requirement for iron in hemoglobin. CD71 expression is upregulated on immature proliferating cells and low levels on resting mature lymphocytes. In addition to this general function, CD71 appears to play a costimulatory role in T cell activation and receptor for IgA1 antibodies.Synonyms: TfR1, Transferrin receptor protein 1, p90