Page 7 - Kids and Cancer

blood cells. The most common type in young people is acute
lymphocytic (lymph cell) leukemia (ALL), which accounts for
about 75 percent of leukemias. The other common leukemia is
acute myelogenous (bone-marrow-forming) leukemia (AML), and
it accounts for most of the remaining leukemias. Both leukemias
are called acute because they are fast growing. Both begin in
the blood- forming cells in the centers of bones. New blood cells
(such as red blood cells, white blood cells, and platelets for blood
clotting) are made in the bone marrow by the master cells called
stem cells. Stem cells are undifferentiated cells that can differenti-
ate, or become, specifi c kinds of cells. In the bone marrow, these
stem cells can become lymphocytes (white blood cells), myeloid
cells (bone marrow cells), red blood cells, or platelets.
The white blood cells called lymphocytes are an important
part of the immune system and fi ght infections. The two main
types of lymphocytes are B cells, which make antibodies that sig-
nal the immune system that a germ has invaded, and T cells,
which attack and kill invaders. ALL develops when one maturing
lymphocyte—usually a B cell—is defective. Instead of developing
into a normal B cell, the cancerous cell never matures completely.
The immature cell is called a lymphoblast, and it does not per-
form the function of a white blood cell. It simply grows rapidly and
multiplies.
AML also starts in the bone marrow, but it involves myeloid
cells instead of lymphocytes. AML usually begins with a defec-
tive myeloid cell that should mature into a kind of white blood cell
(other than a lymphocyte), but it can also start with a stem cell
that is differentiating into a red blood cell or a platelet. As in ALL,
the cancerous cells do not function correctly and crowd out nor-
mal cells. AML is also sometimes called acute myeloid leukemia
or acute nonlymphocytic leukemia.
Leukemia cells build up so abundantly that the body does not
have enough normal red blood cells, white blood cells, or plate-
lets to maintain health. Without enough red blood cells, a per-
son is anemic. The body is not getting enough oxygen to all the

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