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Leukemia is characterized by an increase of abnormal leukocytes in bone marrow. The disease can develop when immature blood cells, which have the ability to regenerate and have a growth advantage compared to normal cells, generate a leukemic clone. Hereditary or acquired deficiency of immunological surveillance can contribute to establishing a leukemic clone. Some leukemic cells can grow without the growth factors which normal cells need to multiply, and many leukemia cells react more poorly to feed-back signals which normally inhibit growth and division of cells. A leukemic clone does not grow more explosively than normal cells, but has a greater tendency to continue replicating and lesser tendency to differentiate and be destroyed. A leukemic clone will gradually take over the bone marrow, and in most cases, spread to the blood.

Leukemia is separated into acute and chronic forms based on the degree of maturity of the characteristic cells.

Acute leukemia

Acute leukemia is divided into two main groups:

  • Acute myeloid leukemia (AML) – clonal expansion of immature myeloid cells (granulocytes, monocytes, erythrocytes, and/or megakaryocytes)
  • Acute lymphoblastic leukemia (ALL) – clonal expansion of immature lymphatic cells

Of adult patients diagnosed with acute leukemia, 80% have acute myeloid leukemia and 20% have acute lymphoblastic leukemia.

Chronic leukemia

Chronic leukemia is divided into two main groups:

  • Chronic myeloid leukemia (CML) – clonal expansion in one or more of the hematopoetic cell rows in bone marrow and also often in the spleen, with mature differentiation of the cells. The disease is progressive and without adequate treatment will often develop into acute leukemia after 4-6 years.  
  • Chronic lymphocytic leukemia (CLL) – clonal expansion of lymphocytes with mature phenotype. The disease is heterogenous with a variable disease course, from being an indolent illness without need for treatment or shortening of life to an agressive disease with a fatal ending after 2-4 years. A small group of patients have more rare subgroups, for example hairy cell leukemia or prolymphocyte leukemia.  


Although leukemia is among the most common childhood cancers, it most often occurs in older adults. Leukemia is slightly more common in men than women and represents 3.6% of all new cancer cases in the United States. Approximately 1.5% of men and women will be diagnosed with leukemia at some point during their lifetime. Leukemia is most frequently diagnosed among people aged 65-74. In 2017, it is estimated to be 40,710 new cases of leukemia in the United States (5).


Age-specific incidence of leukemia, 2010–2014.

Source: National Cancer Institute. Bethesda, MD, USA



Incidence of cancer of leukemia, 1975–2014.

Source: National Cancer Institute. Bethesda, MD, USA


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