L

leukapheresis See HEMAPHERESIS.

leukemia A type of CANCERthat affects the BONE
marrow’s production of leukocytes (white BLOOD
cells). Doctors classify leukemia as either myeloid
(sometimes called myelocytic) or lymphocytic
(sometimes called lymphoblastic), depending on
the type of leukocytes affected. Within either clas-
sification leukemia can be acute or chronic. The
four most common types of leukemia are

• acute lymphocytic leukemia (ALL)


• chronic lymphocytic leukemia (CLL)


• acute myeloid leukemia (AML)


• chronic myeloid leukemia (CML)

LEUKEMIA VS. LYMPHOMA

LYMPHOMAis another type of cancer that can

affect the lymphocytes. However, lymphoma is a

cancer of the lymphatic tissues that produce and

store lymphocytes. Leukemia is a cancer of the

BONE MARROWthat alters the development and

proliferation of the lymphocytes that enter the

BLOODcirculation.

There are a number of subtypes within these
classifications, usually identified according to the
affected cell type or its developmental stage.
Though common perception is that leukemia pri-
marily affects children, 10 times as many adults as
children develop this type of cancer. Children are
more likely to develop acute leukemia and adults
over age 60 to develop chronic leukemia, though
either form can occur at any age. Some forms of
childhood leukemia are fully curable and some
forms of adult leukemia are highly manageable.
GENETIC PREDISPOSITIONand certain environmental

factors, such as exposure to industrial chemicals,

pesticides, and RADIATION THERAPYor CHEMOTHERAPY,

appear to increase an individual’s risk for develop-

ing leukemia. However, most of the time doctors do

not know what causes this leukemia to develop.

How Leukemia Develops

All blood cells arise from pluripotent BLOOD STEM

CELLS, “parent” cells within the BONE MARROWthat

have the ability to form into several different

kinds of blood cells. A complex interaction of

genetic encoding, chemicals, proteins, molecular

functions, and physiologic needs determines the

manner in which blood stem cells differentiate

(become specific kinds of cells) and proliferate

(reproduce themselves). At the first level of differ-

entiation, a blood stem cell establishes its lineage

as lymphoid or myeloid. Myeloid stem cells give

rise to erythrocytes, platelets, granulocytes, and

monocytes. Lymphoid stem cells give rise to lym-

phocytes. In leukemia, the stem cells are normal

though something goes awry at the first stage of

differentiation, and one of the lines—lymphoid or

myeloid—produces abnormal cells.

In acute forms of leukemia the bone marrow

accelerates LEUKOCYTE production and releases

immature leukocytes not yet capable of function-

ing as leukocytes. In relatively short time the

immature cells flood the bone marrow, crowding

out other cells. The onset of symptoms with acute

leukemia is generally rapid because the immature

cells the bone marrow releases cannot function

yet are entering the circulation at a rate that

causes them to quickly become dominant in the

blood. In chronic leukemia the bone marrow’s

rate of production is normal and the leukocytes

the marrow releases into the circulation are

mature but defective. The onset of symptoms in

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