Both twin participants will receive a tetanus booster shot, if needed. The non-infected twin will undergo apheresis to collect white blood cells. For this procedure, whole blood is collected, similar to the procedure for donating a unit of blood from a needle in the arm. The blood flows through a cell separator machine where the white cells are removed, and the rest of the blood (red cells, plasma and platelets) is returned to the donor through a catheter in the opposite arm.