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Transferrin, also known as serum transferrin or transferrin receptor protein, is a glycoprotein that plays a crucial role in iron transport within the body. It is synthesized in the liver and is found in high concentrations in blood plasma. Transferrin is responsible for transporting iron from sites of absorption, such as the intestines, to sites of storage, such as the liver and bone marrow, and to sites of utilization, such as erythropoiesis in the bone marrow.

The transferrin molecule consists of a single polypeptide chain with two identical lobes, each containing a high-affinity iron-binding site. Each site can bind one iron atom, allowing transferrin to transport two iron atoms at a time. The iron-binding sites are located in a cleft between the two lobes of the molecule.

Transferrin binds to specific receptors on the surface of cells, such as the transferrin receptor (TfR), which is a transmembrane protein. The transferrin-receptor complex is internalized by the cell via endocytosis, and the iron is released into the cell's cytoplasm. The transferrin molecule, now depleted of iron, is recycled back to the bloodstream, where it can bind additional iron and repeat the cycle.

Transferrin is essential for maintaining iron homeostasis, as it ensures that iron is delivered to cells that require it, while preventing iron from accumulating in tissues where it is not needed. Disorders of transferrin, such as hereditary transferrinemia, can lead to iron overload and related complications.