Blood cell formation, known as hematopoiesis, starts in the bone marrow, the spongy interior of the large bones, with a pool of immature or undifferentiated cells known as pluripotent stem cells, which contain the characteristics of all the major blood cell lines. These cells divide, either producing themselves exactly or producing more specialized cells that contain the characteristics of only one of the two major cell lines: One of these two specialized cells, known as the mixed myeloid progenitor cell, consists of the progenitor cells to the red blood cells, the monocytes and granulocytes (white cells), and the platelets. The other, the lymphoid stem cell, produces the lymphocytes that make up about 30 percent of the circulating white blood cells. The two types of lymphocytes - T cells and B cells - have different functions: The T cells attack and destroy virus-infected cells, foreign tissue, and cancer cells; B cells produce antibodies, which are proteins that help destroy infectious agents. The two types of white cells interact in complex ways to regulate the immune response.

The production process is continual through division and differentiation, with cell characteristics becoming increasingly defined with each division. The result is that cells are "committed" to evolution into one specific cell type, and thus, as mature cells, they are released into the bloodstream at a rate consistent with the body's needs and the death of old cells. The exception is the lymphocytes: These are allowed to leave the bone marrow as immature cells, but they mature in the lymph system (thymus, spleen, lymph nodes, and so forth) before entering the bloodstream. In normal health, immature stem cells or blasts are present in the circulating blood only in very small numbers, and healthy bone marrow does not contain more than 5 percent of the total cell population.

From 6 weeks until about 6 to 7 months in utero, liver and the spleen are the main organs involved in blood formation. Thereafter, the bone marrow takes over. In infants, practically all the bones are involved, but in adults the production is limited to the vertebrae, ribs, sternum, skull, sacrum, pelvis, and the proximal ends of the femur. Because the entire blood cell formation, growth, maintenance, destruction, and replacement cycle is most efficiently organized, it follows that any abnormal reproduction of any type of cell will disrupt the blood cell balance and, in the absence of a self-regulating mechanism, will affect the body's general health.

Every day at least 200 billion red cells, 10 billion white cells, and over 400 billion platelets are produced in the marrow. The averagel life-span of the cells varies according to the cell type, and ranges from 1 to 2 days for white cells, to about 7 days for platelets, to 120 days for red cells. In normal circulating blood there are approximately 1,000 red cells to each white cell. But in leukemia, the normal production of blood cells fails. In all but its rarest form, one or more of the types of white blood cells reproduces abnormally and creates useless, immature blast cells or poorly developed cells of inferior quality. These leukemic cells begin to overpopulate the bone marrow, spill into the bloodstream and the lymph system, and infiltrate vital organs and glands, causing them to enlarge and malfunction. Because the bone marrow is severely impaired, it is unable to maintain production of sufficient levels of red cells and platelets. As a consequence, the whole balance of the blood cell population is seriously disturbed, and the body's defense mechanisms provided by the white blood cells and the platelets are rendered ineffective.

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