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 relatively small 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.