Many cell membranes carry receptors special for transferrine (sTfR). Transferrine binds to tjose receptors and is taken into the cells via receptor-mediated endocytosis. The acidic pH of lysosomes makes the iron leave the protein. The free iron passes through the cytoplasm. Apotransferritine is not katabolised in the lysosomes, it travelles to the plasma membrane with its receptor, leaves the receptor there, turns bact to the plasma, and binds a new iron in order to carry it to other cells which they need it.

During the endocytical cycle as a result of the externalisation of TfR, a soluble form of it can be detected in the serum. Serum TfR levels are closely related to erythroidal TfR turnover and it is the main determinant of iron need of the cell and erythroidal proliferation rate. Important clinical studies have shown that the plasma sTfR concentration reflects the tissue receptor mass. Since it is not an acute phase reactant, it is a good indicator in detecting the iron deficiency in inflamatory conditions and chronical diseases. It is also not effected in chronical liver diseases and in patients who take oestrogen theraphy. Thus, it is useful in differentiating the patients with chronical disease anemia and iron deficiency anemia.Serum sTfR is also an important indicator of body iron stores during pregnancy and in infants.

Serum sTfR concentration changes as the rate of erytopoiesis change. Its level is under normal in patients with aplastic anemia, chronical inflamation anemia (for example in rhomatoid arthritis patients), chronical kidney insufficiency and hereditary haemochromatosis; and its level is over normal in patients with autoimmune hemolytical anemia and iron deficiency anemia.