What Is Leukemia?

“Leukemia” is the general term for four different types of blood cancer called:
Acute lymphocytic (lymphoblastic) leukemia (ALL)
Acute myelogenous (myeloid) leukemia (AML)
Chronic lymphocytic leukemia (CLL)
Chronic myelogenous leukemia (CML)

It is important to know that how patients are affected and
treated are not the same for each type of leukemia. These four
types of leukemia do have one thing in common – they begin in
a cell in the bone marrow. The cell undergoes a change and
becomes a type of leukemia cell.

The marrow is really two organs in one. The first is the blood
cell-forming organ. This is the site where myelogenous leukemia
begins. The second is the lymphocyte-forming organ and is a
part of the immune system. It is the site where lymphocytic
leukemia begins.

The leukemia is called “lymphocytic” or “lymphoblastic” if the
cancerous change takes place in a type of marrow cell that forms
“lymphocytes.” The leukemia is called “myelogenous” or
“myeloid” if the cell change takes place in a type of marrow cell
that normally goes on to form red cells, some kinds of white cells
and platelets.

The ways in which patients are affected and how they are treated
are different for each type of leukemia.

“Acute lymphocytic leukemia” and “acute myelogenous
leukemia” are each composed of young cells, known as
“lymphoblasts” or “myeloblasts.” These cells are sometimes
called “blasts.” Acute leukemias progress rapidly without
treatment.

“Chronic” leukemias have few or no blast cells. “Chronic
lymphocytic leukemia” and “chronic myelogenous leukemia”
usually progress slowly compared to acute leukemias.

Doctors do not know the causes of most cases of leukemia. They
do know that once the marrow cell undergoes a “leukemic”
change, it multiplies into many cells. These leukemia cells grow
and survive better than normal cells and, over time, they crowd
out normal cells.

The rate at which leukemia progresses and how the cells replace
the normal blood and marrow cells are different with each type
of leukemia.

In acute myelogenous leukemia (AML) and acute lymphocytic
leukemia (ALL), the original acute leukemia cell goes on to
form about a trillion more leukemia cells. These cells are
described as “nonfunctional” because they do not work like
normal cells. They also crowd out the normal cells in the
marrow; in turn, this causes a decrease in the number of new
normal cells made in the marrow. This further results in low red
cell counts (anemia).

In chronic myelogenous leukemia (CML), the leukemia cell
that starts the disease makes blood cells (red cells, white cells
and platelets) that function almost like normal cells. The number
of red cells is usually less than normal, resulting in anemia. But
many white cells and sometimes many platelets are still made.
Even though the white cells are nearly normal in how they work,
their counts are high and continue to rise. This can cause serious
problems if the patient does not get treatment. If untreated, the
white cell count can rise so high that blood flow slows down
and anemia becomes severe.

In chronic lymphocytic leukemia (CLL), the leukemia cell that
starts the disease makes too many lymphocytes that do not
function. These cells replace normal cells in the marrow and
lymph nodes. They interfere with the work of normal
lymphocytes, which weakens the patient’s immune response. The
high number of leukemia cells in the marrow may crowd out
normal blood-forming cells and lead to a low red cell count
(anemia). A very high number of leukemia cells building up in
the marrow also can lead to low white cell (neutrophil) and
platelet counts.

Unlike the other three types of leukemia, some patients with
CLL may have disease that does not progress for a long time.
Some people with CLL have such slight changes that they
remain in good health and do not need treatment for long
periods of time. Most patients require treatment at the time of
diagnosis or soon after.