Leukemia starts in the blood and bone marrow and occurs when white blood cells in the body mutate (change) and grow uncontrollably. Normal white blood cells help the body fight infections and, when they become old or damaged, they die and are replaced by new healthy cells. However, the damaged cells cannot fight infections well and do not die when they should. As a result, they accumulate in the bone marrow, which 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, increasing their risk for infection.

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. (Not all bones have bone marrow.) Bone marrow is where blood is created and contains immature blood stem cells, more mature blood-forming cells, fat cells and tissues that support cell growth. The immature blood stem cells, known as hematopoietic stem cells, develop into several types of blood cells, including white blood cells, red blood cells and platelets.
  • Platelets are blood cells that group around wounds to form clots and stop bleeding. They also play a part in repairing wounds and creating new blood vessels.
  • Red blood cells (erythrocytes) carry oxygen from the lungs to other parts of the body.
  • White blood cells (leukocytes) help the body fight infection. The five major types of leukocytes are neutrophils, lymphocytes, basophils, eosinophils and monocytes.

Several types of white blood cells exist. For the purpose of understanding leukemia, they can be divided into categories.

  • Granulocytes are small enzyme-containing granules visible under a microscope. They develop from myeloblasts (immature cells found in the bone marrow) into mature cells that help the body fight viruses and bacterial infections. Subtypes of these cells include basophils, eosinophils and neutrophils.
  • Lymphocytes make up lymphoid tissue, which is found in the lymph node, thymus, spleen, tonsils and other parts of the body. Lymphocytes are a major part of the immune system and develop from lymphoblasts (immature cells found in bone marrow) into mature, infection-fighting cells. Subtypes of these cells include B-lymphocytes and T-lymphocytes.
  • Monocytes, the largest of the white blood cells, help protect against infections, remove dead or damaged tissues, destroy cancer cells and boost the immune system. Monocytes are produced in the bone marrow and then enter the blood, where they migrate to the spleen, liver, lungs, and bone marrow tissue.

Figure 1

Types of Leukemia

Leukemia is categorized into four major types — acute lymphocytic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML). The kind of cell the leukemia starts in and how rapidly the leukemia grows determine the type of leukemia.

Leukemia is classified as either acute or chronic, based on whether the mutated cancer cells look more like immature (not fully developed) or mature (similar to fully developed) white blood cells. Acute leukemia cells look similar to immature cells and chronic leukemia cells look similar to healthy, mature cells, but they are unable to fully mature.

  • 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. Acute cells also grow quickly in comparison to slow-growing chronic cells.
  • Chronic leukemia grows slowly and occurs when white blood cells mature partially but not completely. 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.

Leukemia may also be classified as lymphocytic or myelogenous (myeloid) based on the type of cells that mutate in the bone marrow.

  • 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.

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