Acute lymphoblastic leukemia (ALL) is a type of leukemia—a cancer of the bone marrow and blood. This disease is sometimes referred to as acute lymphocytic leukemia or acute lymphoid leukemia.
The American Cancer Society estimates that about 6,200 people are diagnosed with ALL each year in the U.S. While ALL is the most common type of leukemia in children, it is the least common type in adults. Only one-third of ALL cases are diagnosed in adults.
What Is ALL?
In people with ALL, a type of blood stem cell called a lymphoblast begins to function abnormally. Instead of maturing into fully functioning lymphocytes—important immune-system cells—the lymphoblasts multiply out of control, and they don’t die off like normal blood cells do.
The underdeveloped (leukemic) cells can’t carry out their normal infection-fighting functions, and they build up in the bone marrow and blood, crowding out normal, healthy blood cells that the body needs. The reduction in the number of normal blood cells can lead to infection, anemia, and excessive bleeding. The leukemic cells also can travel through the bloodstream to other parts of the body, including the lymph nodes, spleen, liver, central nervous system (brain and spinal cord), and testicles, interfering with the normal function of these organs.
ALL progresses and gets worse quickly if it is left untreated, which makes it important to start treatment soon after diagnosis.
Understanding the Bone Marrow and Blood
To understand leukemia, it helps to have basic knowledge of the bone marrow and how healthy blood cells form and what they do.
Stem cells are cells in the body that have the potential to turn into any kind of cell, such as a skin cell, a liver cell, a brain cell, or a blood cell. Stem cells that turn into blood cells are called hematopoietic stem cells, or blood stem cells. Blood stem cells are mainly found in bone marrow (the soft, spongy tissue inside your bones), but some are also found in circulating blood. When blood cells become old or damaged, they die, and blood stem cells produce new blood cells to replace them.
Blood stem cells produce lymphoid stem cells and myeloid stem cells. Lymphoid stem cells produce lymphoblasts, which in turn produce several types of white blood cells, including lymphocytes and natural killer cells. Myeloid stem cells produce myeloblasts, which turn into white blood cells known as granulocytes, as well as red blood cells and platelets.
- White blood cells (leukocytes) fight infection. The main types of white blood cells are B lymphocytes (B-cells), T lymphocytes (T-cells), natural killer (NK) cells, and granulocytes.
- Red blood cells (erythrocytes) carry oxygen from the lungs to the other parts of the body and take carbon dioxide back to the lungs to be removed.
- Platelets (thrombocytes) make the blood clot and slow or stop bleeding.
What Causes ALL?
Doctors do not know what causes ALL. The disease is more common in children age 5 years old and younger, and the risk increases again after age 50. It is also more common in males than females and in whites than African-Americans.
The few known risk factors that may increase your odds for developing ALL include:
- Being exposed to high levels of radiation
- Being exposed to certain chemicals, such as chemotherapy or benzene
- Having certain viral infections, although this is more common outside the United States
- Having certain genetic conditions, such as Down syndrome
- Having an identical twin who develops ALL as an infant or young child
Keep in mind that many people who develop ALL have none of the risk factors and most people with the risk factors do not develop the disease.