According to the American Cancer Society, about 54,300 people are diagnosed with leukemia in the United States each year. More than 90 percent of all leukemia cases are diagnosed in adults over age 20.
What Is Leukemia?
Unlike many cancers, leukemia rarely forms solid tumors. Instead leukemia is cancer of the bone marrow and blood. In a person with leukemia, the bone marrow produces abnormal (leukemic) blood cells. The leukemic cells divide and multiply but don’t go through the normal process of maturing and eventually dying, like healthy blood cells do.
The underdeveloped leukemic cells can’t carry out their normal functions, and they build up in the bone marrow and blood, crowding out normal, healthy blood cells that the body needs. The reduction in normal blood cells can lead to a number of symptoms, including 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.
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.
- 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.
Types of Leukemia
Leukemias are named for the type of blood stem cell—lymphoid or myeloid—that is affected and how quickly the disease develops and progresses. Acute leukemias grow rapidly, leading to symptoms, and they worsen quickly without treatment. Chronic leukemias are slower to develop.
There are four main types of leukemia in adults.
Acute Lymphoblastic Leukemia (ALL)
Also known as acute lymphocytic leukemia or acute lymphoid leukemia, ALL affects the lymphoblasts. In the U.S., about 6,200 people are diagnosed with ALL annually; about two-thirds of these are children. Read more about ALL in adults.
Acute Myeloid Leukemia (AML)
Also known as acute myelogenous leukemia or acute myelocytic leukemia, AML affects the myeloid cells. About 20,800 new cases of AML are diagnosed each year in this country, mostly in adults over age 45. Read more about AML in adults.
Chronic Lymphocytic Leukemia (CLL)
Also known as chronic lymphoid leukemia or chronic lymphoblastic leukemia, CLL affects the lymphocytes. CLL accounts for one-third of new leukemia cases in adults. Experts expect about 14,600 people per year in the U.S. to be diagnosed with CLL. CLL does not occur in children. Read more about CLL.
Chronic Myeloid Leukemia (CML)
Also known as chronic myelogenous leukemia or chronic myelocytic leukemia, CML affects the granulocytes. About 6,700 people in the United States are diagnosed with CML annually. This type is extremely rare in children. Read more about CML in adults.
In addition, there are several other types of leukemia and related blood disorders, such as hairy cell leukemia (HCL) and chronic myelomonocytic leukemia (CMML).
Leukemias are further grouped into subtypes, phases, and risk categories based on:
- Cytogenetic (chromosomal) and molecular abnormalities associated with your disease
- How far your leukemia has spread
- Your treatment status
- Your response to treatment
Your doctor uses this information, along with other prognostic factors such as your age, general health, and gender, to plan your treatment and predict the outcome.
What Causes Leukemia?
Doctors do not know what causes some blood cells to become leukemic. Often changes or mutations in specific genes or chromosomes are seen in people with leukemia. While leukemia is the most common cancer in children, the majority of cases of leukemia (more than 90 percent) are in adults over age 20. Learn more about pediatric leukemia.
For most people with leukemia, there are no obvious reasons why they developed the disease. Some factors that may increase risk include:
- Being exposed to certain viruses, chemotherapy, or radiation
- Being exposed to certain chemicals, such as cigarette smoke, herbicides, or pesticides
- Having certain genetic conditions, such as Down syndrome
- Having a family history of blood cancers or disorders
Race and ethnicity, gender, and age also influence risk.
However, most people who develop leukemia have no risk factors, and most people with the risk factors do not develop the disease.