Acute lymphocytic leukemia, the most common form of childhood leukemia, is one of the many different types of leukemia which may occur in a child. Leukemia is defined as "a malignancy of the blood and blood-forming tissues." (Bain 9) To fully understand how leukemia develops, one must know about the different cells of the blood and how they are developed.

Blood cells originate in the bone marrow (tissue primarily present in the hollow centers of the bone). Here, the stem cell (the most immature type of cell) differentiates to form red blood cells, white blood cells, and platelets. Red blood cells (or erythrocytes) primarily carry oxygen to the body tissues from the lungs and carbon dioxide form the body tissues to the lungs. White blood cells (or leukocytes) aid the body in fighting off disease and infection. Finally, platelets (or thrombocytes) aid in the control of bleeding by releasing the substance that triggers blood clotting.

The stem cells which differentiate into WBCs further specialize into granulocytes and monocytes (responsible for engulfing and destroying foreign material such as bacteria) and lymphocytes. Lymphocytes are the major cells of the immune system, and they consist of B cells ("cells that produce antibodies- protein molecules that allow the body to recognize and eliminate foreign invaders") and T cells (cells which carry out many different immunologic functions such as helping and suppressing the immune response).

Acute lymphocytic leukemia "is caused by a rapid proliferation of immature lymphocytes, which would normally have developed into mature T-cells or B-cells." The immature cells are also called lymphoblasts, which is why this type of leukemia is also known as acute lymphoblastic leukemia.

Sources:
Bain, Lisa J. A Parent’s Guide To CHILDHOOD CANCER. New York: Dell Publishing, 1995.
“Leukemia.” World Book Encyclopedia. Volume 12, pps. 191-192. 1984.
Keene, Nancy. Childhood Leukemia. Sebastopol: O’Reilly & Associates, 1997.

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