Stem cells are small, round cells with a squat nucleus and scant surrounding cytoplasm. Although unremarkable in appearance, stem cells can perform what have been called "acts of biological resurrection." Whereas other types of cells in the body have a limited lifespan and die after dividing their endowed number of times, a stem cell can reproduce forever. The stem cell is immortal (in cellular terms).
A stem cell can forgo immortality and turn into an ordinary blood cell, a red blood cell (an erythrocyte), a white blood cell (a leukocyte), or a large cell (a megakaryocyte) that fragments into the platelets needed for blood to clot.
A limited number of stem cells can miraculously repopulate the whole bone marrow, provide an endless supply of stem cells, reconstitute the entire repertory of blood cells, and restore the immune system.
To obtain the stem cells, the stem cells are lured out of the bone marrow by a special regimen of drugs. The blood is then filtered through a machine and the stem cells are skimmed off. The removal of the cells is termed pheresis or apheresis (from the Greek "aphairesis" for removal).
Before the transplant is done, the patient receives high-dose chemotherapy and/or radiation therapy to destroy diseased cells (the leukemic cells, lymphoma cells, solid tumor cells, the diseased immune system cells in scleroderma, etc.) The stem cells are then returned to the patient, where they can produce new blood and immune cells and replace the cells destroyed by the treatment. The stem-cell preparation is infused into a vein and, once there in the blood stream, the stem cells act like homing pigeons and head straight for the bone marrow space.