Introduction to Leukemia

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As defined by the National Cancer Institute (NCI) (Leukemia definition), Leukemia is
“Cancer that starts in blood-forming tissue, such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the bloodstream.”

Leukemia is cancer of blood-forming tissues. White blood cells may be produced in excessive amounts and are unable to work properly, which weakens the immune system. Leukemias are primarily limited to bone marrow involvement; however, with more extensive involvement, myeloid sarcomas, or leukemia cutis may be diagnosed on the skin. These are not new primaries, but a manifestation of the leukemia.

Blood cells are formed in the bone marrow, the soft, spongy center of bones. New (immature) blood cells are called blasts. Some blasts stay in the marrow to mature. Some travel to other parts of the body to mature.

Normally, blood cells are produced in an orderly, controlled way, as the body needs them. This process helps keep us healthy. When leukemia develops, the body produces large numbers of abnormal blood cells. In most types of leukemia, the abnormal cells are white blood cells. The leukemia cells usually look different from normal blood cells, and they do not function properly.

Unlike normal blood cells, leukemia cells don’t die when they become old or damaged. Because they don’t die, leukemia cells can build up and crowd out normal blood cells. The low level of normal blood cells can make it harder for the body to get oxygen to the tissues, control bleeding, or fight infections. Leukemia cells can also spread to other organs, such as the lymph nodes, spleen and brain.

Leukemia patients may present with anemia, which is a shortage of red blood cells. The normal white blood cell production is decreased, and the abnormal white blood cells do not function properly to prevent infection.

Patients may also present with thrombocytopenia, which is low blood platelets resulting from excessive white blood cells crowding out the blood platelets. Since platelets control blood clotting, the patient may present with symptoms of excessive bruising, bleeding, nosebleeds and bleeding from the gums.

Acute and Chronic Leukemia

Type of Leukemia Definition Examples
Acute
  • Patients symptomatic at diagnosis (anemia, fatigue, bruise easily)
  • Develops quickly
  • Abnormal cells (blasts) can’t carry out function of normal white blood cells
  • Bone marrow may show elevated levels of leukemia cells and low levels of normal blood cells
  • Acute basophilic leukemia
  • Acute leukemia, NOS
  • Acute megakaryoblastic leukemia
  • Acute monoblastic and monocytic leukemia
  • Acute myeloid leukemia
  • All variants of Acute myeloid leukemia (AML)
Chronic
  • Patient may be asymptomatic at time of diagnosis
  • Develops slowly
  • Leukemia cells work almost as normal white blood cells
  • First sign of illness may be abnormal results on a routine blood test
  • Chronic eosinophilic leukemia
  • Chronic lymphocytic leukemia
  • Chronic myeloid leukemia

Four Most Common Types of Leukemia

Acute lymphoblastic leukemia (ALL) affects lymphoid cells and grows quickly. Leukemic blast cells usually collect in the bone marrow and blood. ALL is very common in children.

Acute myeloid leukemia (AML) affects myeloid cells and grows quickly. Leukemic blast cells collect in the bone marrow and blood.

Chronic lymphocytic leukemia (CLL) affects lymphoid cells and usually grows slowly. Blood tests show an increase in the number of white blood cells. The abnormal cells work almost as well as the normal white blood cells.

Chronic myeloid leukemia (CML) affects myeloid cells and usually grows slowly at first. Blood tests show an increase in the number of white blood cells. The abnormal blood cells function fairly well. There may be a small number of leukemic blast cells in the bone marrow.

  • CML has four different phases, three of which are distinguished by the number of blasts in the peripheral blood or the bone marrow
CML Phase Definition
Chronic Less than 10% blasts in the peripheral blood or bone marrow
Accelerated 10% to 19% blasts in either the peripheral blood or bone marrow
Blastic 20% or more blasts in the peripheral blood or bone marrow
Relapsing Evidence of progression of disease from a stable remission which may include the following:
  • Increasing myeloid or blast cells in the peripheral blood or bone marrow
  • Cytogenetic positivity for CML when previously cytogenetic-negative
  • FISH positivity for BCR/ABL (breakpoint cluster region/Abelson) translocation when previously negative

See the Hematopoietic Database or the Hematopoietic Manual (Appendix B) for more information on the types of Leukemias.

Updated: December 2, 2025