What is leukemia?

Leukemia is a cancer that starts in blood stem cells. Stem cells are basic cells that develop into different types of cells that have different jobs.

Blood stem cells develop into either lymphoid stem cells or myeloid stem cells.

  • Lymphoid stem cells develop into lymphocytes, a type of white blood cell. Lymphocytes help fight infection and destroy abnormal cells. The 3 types of lymphocytes are B cells, T cells and natural killer (NK) cells.
  • Myeloid stem cells develop into red blood cells, granulocytes, monocytes or platelets. Red blood cells carry oxygen to all tissues of the body. Granulocytes and monocytes are types of white blood cells that destroy bacteria and help fight infection. Platelets form clots in damaged blood vessels to stop bleeding.

As the stem cells of the blood develop, they become blast cells (blasts), which are immature blood cells. In leukemia, there is an overproduction of blast cells. These blast cells develop abnormally and don’t develop into mature blood cells. Over time, the blast cells crowd out normal blood cells so that they can’t do their jobs. When leukemia is diagnosed, these blast cells may be called leukemia cells.

There are many different types of leukemia. They are grouped based on the type of blood stem cell they developed from. Lymphocytic leukemias (also known as lymphoblastic leukemias) develop from abnormal lymphoid stem cells. Myelogenous leukemias develop from abnormal myeloid stem cells.

The types of leukemia are further grouped based on how quickly the leukemia develops and grows. Acute leukemias start suddenly, developing within days or weeks. Chronic leukemias develop slowly over months or years.

The 4 main types of leukemia are acute lymphocytic leukemia (ALL), acute myelogenous leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myelogenous leukemia (CML).

In adults, CLL and AML are the most common leukemias. There are many different subtypes of leukemia.

Diagram of development of blood cells
Diagram of development of blood cells

Expert review and references

  • Kurtin SE . Leukemia and myelodysplastic syndromes. Yarbro, CH, Wujcki D, & Holmes Gobel B. (eds.). Cancer Nursing: Principles and Practice. 7th ed. Sudbury, MA: Jones and Bartlett; 2011: 57: pp. 1369-1398.
  • Biology of the immune system. Porter, R. S., Kaplan, J.L. & Homeier, B.P., et al. (Eds.). The Merck Manuals Online Medical Library: Home Edition for Patients and Caregivers. Whitehouse Station, N.J.: Merck Research Laboratories, a division of Merck & Co, Inc; 2008.

The blood and bone marrow

Bone marrow is the spongy, liquid substance in the centre of some bones. Its main function is to produce blood cells. It constantly produces blood cells to meet the body’s needs.

Rare lymphocytic leukemias

Leukemias that start from lymphocytes are classified based on where the abnormal lymphocytes are found. Learn about rare lymphocytic leukemias.

Hairy cell leukemia

Hairy cell leukemia is an uncommon type of chronic leukemia. Learn about symptoms, diagnosis and treatment of hairy cell leukemia.

Blood disorders

Blood disorders develop when the body makes too many or not enough blood cells. Learn about some of these blood disorders.

Polycythemia vera

Polycythemia vera (PV) develops when the body makes too many red blood cells and sometimes too many white blood cells or platelets. Learn about PV.

Idiopathic myelofibrosis

Idiopathic myelofibrosis (IM) develops when blood cells don’t develop normally. Learn about symptoms, diagnosis and treatment of IM.

Essential thrombocytosis

Essential thrombocytosis (ET) develops when the body makes too many platelets. Learn about symptoms, diagnosis and treatment of ET.

Chronic eosinophilic leukemia

Chronic eosinophilic leukemia (CEL) develops when the body makes too many eosinophils (a type of white blood cell). Learn about CEL.

Chronic neutrophilic leukemia

Chronic neutrophilic leukemia (CNL) develops when the body makes too many neutrophils (a type of white blood cell). Learn about CNL.

Myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are a group of diseases in which the bone marrow doesn’t make enough healthy mature blood cells. Learn about MDS.

Myelodysplastic/myeloproliferative neoplasms

Myelodysplastic/myeloproliferative neoplasms have features of myelodysplastic syndromes and myeloproliferative neoplasms. Learn about the most common types.