Leukemia is a blood cancer that starts in the blood and bone marrow. It is the abnormal growth and accumulation of blood cells in the body. Unlike normal blood cells, these abnormal cells cannot fight infections well. The abnormal cells may slow down or prevent normal body functions, including the production of healthy blood cells. People with leukemia often have low numbers of healthy white blood cells, red blood cells and platelets. As a result, risk of infection is high and transfusions of red blood cells and platelets may be needed.
Leukemia is categorized into four major types. The kind of cell the leukemia starts in and how rapidly the leukemia grows determine the type of leukemia. Leukemia can also involve lymph nodes, the liver, the spleen, the brain and, rarely, the skin.
Bone marrow and types of blood cells
Learning more about the parts of blood and bone marrow and the roles they play can help you better understand leukemia (see Figure 1).
Bone marrow is the soft, spongy center of some bones that contains immature blood stem cells, mature blood-forming cells, fat cells and tissues that support cell growth. Blood cells are made in the bone marrow.
Platelets are blood cells that group around wounds to form clots and stop bleeding.
Red blood cells (erythrocytes) deliver oxygen to the body.
White blood cells (leukocytes) help the body fight infection.
Several types of white blood cells exist, and they can be divided into two main categories for the purpose of understanding leukemia:
Granulocytes are cells with enzyme-containing granules visible under a microscope. They develop from myeloblasts (immature cells found in bone marrow) into mature, infection-fighting cells. Subtypes of these cells include basophils, eosinophils and neutrophils.
Lymphocytes are the primary cells in lymphoid tissue or “glands,” a major part of the immune system. They develop from lymphoblasts (immature cells found in bone marrow) into mature, infection-fighting cells. Subtypes of these cells include B lymphocytes, T lymphocytes, and natural killer (NK) lymphocytes.
Types of leukemia
The four major types of leukemia are acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML). They are categorized by how fast they grow and by the type of cell in which they start.
Acute leukemia grows quickly and occurs when the number of immature white blood cells increases rapidly, preventing bone marrow from making normal blood cells. Treatment should begin as soon as possible because these fast-growing cells can quickly become life-threatening. People with acute leukemia have symptoms such as fatigue, fever, easy bleeding or bruising, weight loss, pain and frequent infections.
Chronic leukemia grows slowly and occurs when white blood cells mature partially but not completely. These abnormal cells do not fight infection as well as normal cells, and they accumulate and crowd out normal cells because they survive longer than normal cells. Progression of chronic leukemia varies among patients. In some cases, chronic leukemia can progress within weeks or months, and in others, it may take years for the disease to progress. People with chronic leukemia may have no symptoms or mild symptoms such as fatigue at the time of diagnosis.
In addition to being categorized as acute or chronic, the four major types are categorized as either lymphocytic or myeloid leukemias. This categorization depends on the kind of cell in which the leukemia starts.
Lymphocytic leukemia begins in cells that become lymphocytes. Lymphocytic leukemias are also sometimes called lymphoid or lymphoblastic leukemias.
Myeloid leukemia begins in early myeloid cells, which become white blood cells (excluding lymphocytes), red blood cells or cells that make platelets. Myeloid leukemias are sometimes called myelogenous, myelocytic or myeloblastic leukemias.
Risk factors for leukemia
The exact cause of leukemia is unknown, but certain factors can increase the risk of its development. These factors include exposure to high levels of radiation and certain chemicals, certain chemotherapy drugs, and some medical conditions, including inherited disorders involving chromosomal and blood abnormalities.
Symptoms of leukemia
Symptoms of leukemia vary among patients and depend on the number of leukemia cells in the bloodstream, the type of leukemia and the area of the body where leukemia cells collect. If the brain is affected, symptoms may include vomiting, confusion, headaches, seizures or loss of muscle control. Leukemia may also affect the digestive tract, heart, lungs, kidneys or testes.
Common symptoms include fever, chills, night sweats, swollen or tender lymph nodes, frequent and recurrent infections, weakness, fatigue, easy bruising and bleeding, unexplained weight loss, loss of appetite, discomfort or swelling in the stomach area and bone or joint pain.