Transferrin is a glycoprotein that binds iron very tightly but reversibly. Although iron bound to transferrin is less than 0.1% (4 mg) of the total body iron, it is the most important iron pool, with the highest rate of turnover (25 mg/24 h). Transferrin has a molecular weight of around 80 kiloDaltons and contains 2 specific high-affinity Fe(III) binding sites. The affinity of transferrin for Fe(III) is extremely high (10²³ M^-1 at pH 7.4 but decreases progressively with decreasing pH below neutrality.When not bound to iron, it is known as "apo-transferrin".

The rate of iron release from HepG2 liver cells was increased not only by extracellular apotransferrin, but also by diferric transferrin, in a non-additive, concentration-dependent manner and to a similar magnitude. This suggests that rapid equilibration between receptor-mediated uptake and the release process determines net iron retention by the liver.