"Keeping a normal white blood cell count is one of the very basic necessities for good health. Your body uses white blood cells to fight off viruses, bacteria, and all sorts of potentially harmful foreign invaders. If your levels fall for some reason, you're much more prone to illness. Chemotherapy and radiation are probably the most well-known offenders causing low white cell counts, but levels can fall for any number of reasons. This is where lithium comes in: Researchers have found that it can increase white cell numbers again in people whose levels fell due to radiation and/or chemotherapy (and even if the person continues those treatments).
Lithium achieves these effects by stimulating the stem cells in bone marrow, which then turn into platelets and white blood cells. I've observed that low dose lithium (5-10 milligrams twice daily) will also usually raise a low count to normal even if radiation and chemotherapy aren't the culprits."