What is acute lymphoblastic leukemia?

Updated March 18, 2026

Although acute lymphoblastic leukemia is a rare type of blood cancer, it is the most common form of leukemia in children. It also affects adults, who comprise 4 out of every 10 cases of acute lymphoblastic leukemia. The risk of developing acute lymphoblastic leukemia is highest in those under 5 years old or over 50 years old. Acute lymphoblastic leukemia usually develops quickly, over the course of days or weeks, and requires rapid treatment.

Which blood cells are affected in acute lymphoblastic leukemia?

To understand which cells are affected in acute lymphoblastic leukemia, it helps to look at blood composition. Blood is broadly made up of four components:

1. Red blood cells, which carry oxygen and give blood its distinctive color.
2. White blood cells, which fight infection.
3. Platelets, which are small cells involved in clotting and prevention of excessive bleeding.
4. Plasma, the straw-colored fluid in which blood cells are suspended.

Under normal conditions, these components exist in relatively stable amounts in relation to each other. However, in acute lymphoblastic leukemia, the number of white blood cells is no longer controlled correctly. White blood cells are produced more rapidly than normal within the bone marrow and are released before they are fully mature (these cells are known as lymphoblasts). This means that although there are more white blood cells in the blood, they do not function correctly.

This overproduction of white blood cells reduces the amount of red blood cells and platelets that are produced, and this causes many of the symptoms of acute lymphoblastic leukemia.

References

1. Cancer Research UK. What is acute lymphoblastic leukaemia (ALL)? https://www.cancerresearchuk.org/about-cancer/acute-lymphoblastic-leukaemia-all/about. Updated May 8, 2024. Accessed Mar 18, 2026.
2. American Cancer Society. Key Statistics for Childhood Leukemia. https://www.cancer.org/cancer/types/leukemia-in-children/key-statistics.html. Updated Jul 22, 2025. Accessed Mar 18, 2026.
3. American Cancer Society. Key statistics for acute lymphocytic leukemia (ALL). https://www.cancer.org/cancer/acute-lymphocytic-leukemia/about/key-statistics.html. Updated Jan 13, 2026. Accessed Mar 18, 2026.