When healthy people donate cells by apheresis, a blood thinner called citrate is added to prevent the blood from clotting in the apheresis machine. Citrate works by reducing calcium in the blood. When the blood is returned to the donor, citrate from the machine is also returned, lowering the donor's calcium levels. As a result, donors often feel tingling around the mouth, hands, and feet. Some of these symptoms can be prevented by giving calcium intravenously (through a vein) during the procedure. Even with the added calcium, however, some donors still have symptoms. Magnesium levels are also lowered by citrate, but it is not known if this causes symptoms. This study will examine whether the decrease in magnesium levels also contributes to the side effects of apheresis and whether magnesium replacement can reduce these symptoms.