White blood cells defend the body against infection. Blood cells grow in the bone marrow. They are supposed to be fully grown when they enter the bloodstream. Leukemia causes white blood cells to lose their ability to mature fully. Immature cells take over the marrow and prevent it from making enough of the red and white blood cells and platelets that your body needs.
Initial symptoms are vague.
Usually you just don't feel very well. Later, symptoms include fatigue, fever, dizziness, enlargement of the spleen, swollen lymph nodes, and frequent infections. Many patients live for years with chronic leukemia, often with little or no discomfort. Sometimes treatment is postponed if you have no symptoms. When the white blood cell count rises above a certain level, or symptoms appear, treatment usually involves chemotherapy.