Participants will undergo leukapheresis to collect white blood cells. This procedure allows collection of larger numbers of cells than would be possible with simple blood drawing. For the procedure, blood is removed through a needle in the vein of one arm and spun in a machine that separates the blood into its components. The white cells are extracted and the rest of the blood is re-infused through the same needle or through a needle in the other arm. The procedure takes approximately 1-3 hours, depending on the amount of white cells being collected. A maximum of three leukapheresis procedures are done. If additional procedures are required, the patient will sign a new consent form. Procedures will be limited to no more than three times a year, or once every 4 months.