Leukemia is a group of cancers that originate in white blood cells called leukocytes. These cells form in the bone marrow -- the soft, spongy inner portion of certain bones.
About 44,790 new cases of leukemia are diagnosed in the United States each year. Acute myelogenous leukemia (AML) and chronic lymphocytic leukemia (CLL) are the most common types in adults. Chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (ALL) are less common.
Although rarely diagnosed in adults, ALL represents about 85 percent of all childhood leukemias, while about 15 percent of pediatric leukemias are AML. For information about pediatric ALL and AML, visit the pediatric cancer care section of our Web site.
How Leukemia Develops
Normal blood-cell development begins in the marrow with the formation of hematopoietic stem cells. These primitive cells are capable of developing into the full range of blood cells -- red and white blood cells and platelets -- each of which makes important contributions to how the body functions. Normal, healthy white blood cells, or leukocytes, have a very short life span -- sometimes only a few hours long -- and are continuously replenished in the bone marrow. They proceed through their life cycles in an orderly way, and when they die they are replaced by new cells.
Leukemia develops when the genetic material, or DNA, of a white blood cell is damaged and the cell becomes malignant and capable of uncontrolled growth. The diseased cells cannot mature beyond an early stage in their life cycles, so they never develop into functional cells. Because they do not mature, they become immortal and live indefinitely. The diseased cells -- called blasts -- eventually take over the bone marrow and displace the normal red and white blood cells and platelets.
As the numbers of normal cells decline, patients may develop anemia, a low level of oxygen-carrying red blood cells; infections caused by low counts of disease-fighting white blood cells; and bruising and bleeding, resulting from low levels of platelets, the blood component crucial to blood clotting and wound healing. Leukemic cells may also invade the liver, spleen, lymph nodes, and other organs.
Types of Leukemia
Blood cells can become malignant at any stage in their development. The leukemia cells that result carry many characteristics of the cell from which they began. Most leukemias develop from one of two types of white blood cells, lymphocytes or myelocytes, and are classified as lymphocytic leukemia or myeloid leukemia, respectively.
Lymphocytic Leukemia
CLL and ALL are lymphocytic leukemias that develop in either T lymphocytes (T cells), B lymphocytes (B cells), or natural killer (NK) cells. Each of these cell types has a specialized role in the immune system, which includes producing antibodies and other substances that fight infections.
Myeloid Leukemia
AML and CML are myeloid leukemias that arise when the genetic material in specialized white blood cells called granulocytes and monocytes becomes altered or mutated, and the body begins producing poorly functioning copies of these cells.
Granulocytes get their name from the enzyme-packed granules they carry inside. They release these enzymes when they encounter invading bacteria or fungi. Monocytes are a precursor to cells called macrophages, which engulf and destroy bacteria and fungi.
Acute or Chronic Leukemia
Physicians classify leukemias according to whether they are acute or chronic. In acute leukemias, the malignant cells, or blasts, are immature cells that are incapable of performing their immune system functions. The onset of acute leukemias is rapid.
Chronic leukemias develop in more mature cells, which can perform some of their duties but not well. These abnormal cells usually increase at a slower rate, so the disease may develop more slowly than in acute leukemia.
Details of the Four Main Types of Leukemia
