This study will evaluate changes in T cells-infection-fighting white blood cells of the immune system-in people given a drug called G-CSF before donating stem cells (immature bone marrow cells) for transplantation. Originally, bone marrow cells were obtained using a needle inserted into the hipbone of the donor. However, when it was discovered that immature bone marrow cells also circulate in the blood, scientists began to develop techniques to collect quantities of these cells from the blood instead of the bone. G-CSF is a blood cell growth factor that pushes stem cells from the bone marrow into the bloodstream. Stem cell donors are given this drug 5 to 6 days before donation to make more cells available in the circulating blood for collection.