Transferrin is a glycoprotein with a relative mass of about 80,000 Da containing two high affinity binding sites. Plasma transferrin is involved in transporting iron from the alimentary tract to the tissues of the body. Its extremely high binding affinity (1022 dm3mol-1) means that it can also outcompete many microorganisms for iron, acting as a bacteriostat.

The two metal-binding sites on transferrin are similar but not identical and function independently. In the absence of iron transferrin is capable of binding other metal ions such as aluminium, chromium, cobalt, copper, manganese, vanadium as well as plutonium and europium. Similar proteins are found in milk (lactoferrin) and eggs (ovotransferrin).