Transferrin is a major glycoprotein in the blood that is responsible for transporting iron from liver tissue (where iron is primarily stored) to cells in other tissues.All growing cells have receptors on their surfaces for iron-binding transferrin, which binds to iron at neutral pH and then enters the cell through endocytosis. Inside the cell, in the acidic environment of the endosome, transferrin releases iron, but still binds to the membrane receptor and returns to the plasma membrane with the receptor. When the extracellular environment becomes neutral, transferrin breaks away from the receptor, binds freely to iron, and begins the cycle again. In effect, transferrin shuttles between the extracellular fluid and the endosome, avoiding the lysosomes and rapidly delivering the iron needed for cell growth.