Ingested iron is absorbed primarily from the intestinal tract and is temporarily stored in the mucosal cells as Fen 3 + ‑ferritin, a complex of ferric hydroxide‑ferric phosphate attached to the protein apoferritin. On demand, iron is released from the mucosal cells into the blood as Fe2 3+ ‑transferrin in equilibrium with a very small amount of free Fe 3+ .
Transferrin is the plasma iron transport protein that binds iron strongly at physiological pH. Transferrin is generally only 25 to 30 percent saturated with iron. The additional amount of iron that can be bound is the unsaturated iron binding capacity (UIBC). The sum of the serum iron and UIBC represents the total iron binding capacity (TIBC).