Transfer of iron from one organ to another is done by the plasma protein apotransferrin. This protein, which is a β1-globin with a molecular weight of 75 kDa, has two iron binding sites, each site can bind to a ferric iron ion Fe3+ together with a bicarbonate ion (HCO3-). These two sites have a high binding affinity for ferric iron ion (Fe3+) at physiological pH, but with a decrease in pH, their binding affinity decreases and allows the release of iron inside the cellulose organelles. Apotransferrin-Fe³+ complex is called transferrin. The iron in the serum binds to transferrin, but since only one third (about 25-30%) of the iron-binding sites on transferrin are occupied by Fe³+ in the normal state, this protein has an unfilled capacity. A consideration for Fe³+ binding is the Unsaturated Iron-Binding Capacity of transferrin or (Unsaturated Iron-Binding Capacity) UIBC indicates the available (empty) iron binding sites in the serum. Total Iron-Binding Capacity (TIBC) represents the amount of iron needed to completely saturate (100%) transferrin, in other words, TIBC is the maximum amount of iron that can bind to serum proteins, especially apotransferrin. Transferrin is made in the liver and its plasma concentration is regulated by the level of iron, in iron deficiency, the level of transferrin increases but its saturation percentage decreases, on the contrary, in anemia caused by chronic diseases, the concentration of transferrin may be normal. or decrease, but the percentage of transferrin saturation increases. Transferrin is one of the negative reactive proteins of the acute phase, which means that its level decreases in various acute inflammatory reactions. The amount of serum TIBC varies in disorders related to iron metabolism and often increases in iron deficiency and decreases in chronic inflammatory disorders, malignancies and hemochromatosis.