THE PLASMA-TO-CELL CYCLE OF TRANSFERRIN*

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Abstract
A study was made of the interaction between human blood cells and human transferrin labeled with I131. Transferrin was found to attach specifically to immature red cells. Transferrin exists in a dynamic equilibrium between that in solution, as in plasma, and that attached to immature red cells. The avidity of Fe-laden transferrin for these cells exceeds that of Fe-free transferrin. These findings suggest the following mechanism for delivering Fe to the immature red cell Plasma transferrin containing Fe selectively attaches to the surface of immature red cells; this attachment permits the active removal of Fe from transferrin by these cells; the transferrin, freed of Fe, is preferentially displaced by other molecules of plasma transferrin laden with Fe; the displaced transferrin then acquires more Fe from storage sites and the plasma-to-cell cycle is repeated.
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