All participants will have a complete medical history and physical examination, blood tests and a tetanus booster shot, if indicated. The non HIV-infected twin will then undergo lymphapheresis to collect lymphocytes. In this procedure, whole blood is collected through a needle placed in an arm vein. The blood circulates through a machine that separates it into its components. The lymphocytes are then removed, and the red cells and plasma are returned to the donor, either through the same needle or through a second needle placed in the other arm.