transferin

Transferrin is a glycoprotein produced primarily by the liver that serves as the main iron-transport protein in the blood. It is a roughly 80 kDa protein with two high-affinity iron-binding sites, and it exists in iron-free form (apotransferrin) or iron-saturated form (holotransferrin). Transferrin binds ferric iron (Fe3+) and keeps it soluble for transport to tissues.

Transferrin delivers iron to cells through transferrin receptors, notably TfR1 and TfR2, which are expressed on many cell types including developing erythroid cells. The transferrin-iron complex binds to these receptors, is internalized by endocytosis, and in the acidic endosome iron is released for cellular use or storage. The receptor and apotransferrin then recycle back to the cell surface.

Transferrin is part of systemic iron homeostasis. Serum transferrin concentration provides transport capacity and is often inferred from the total iron-binding capacity (TIBC). Transferrin saturation, calculated as serum iron divided by TIBC, indicates how much of transferrin’s iron-binding sites are occupied. Transferrin is a negative acute-phase reactant, and its levels can decrease during inflammation or malnutrition, while they often rise in iron deficiency.

Disorders and clinical use: In iron deficiency, transferrin levels and TIBC typically rise, whereas transferrin saturation falls. In chronic liver disease or nephrotic syndrome transferrin levels may be reduced. A rare hereditary condition called transferrin deficiency or transferrinemia disrupts iron transport and can lead to iron overload; management is specialized and may involve experimental or supportive therapies.

See also: transferrin receptor, ferritin, hepcidin.