Types of leukemia
There are multiple types of leukemia, classified based on the type of white blood cell the cancer arose from and how fast it is progressing. Leukemia begins in the bone marrow, located in the soft center of the body's larger bones, which makes precursors of red blood cells, platelets, and white blood cells. These immature blood cell precursors grow and mature in the bone marrow until being released into the bloodstream.
Two categories of immature precursor cells produce white blood cells: myeloid cells and lymphoid cells. Myeloid cells produce red blood cells, platelets, and several white blood cells types, known as granulocytes. Granulocytes circulate in the blood and fight infections by killing and digesting bacteria. Lymphoid cells develop into lymphocytes, another type of white blood cell that is found in both the blood and the lymphatic system. They coordinate the body's immune response and a subset of lymphocytes produce antibodies.
(Other types of cancer affect lymphocytes but occur in the lymphatic system, not the bone marrow. These are called lymphomas and are diagnosed and treated differently. They are discussed in the Lymphoma article.)
Leukemia is categorized based on whether the abnormal white blood cells are from lymphoid cells or myeloid cells. A further classification is based on if the cancer is fast developing and rapidly fatal if not treated (acute) or more slowly developing (chronic). That leads to four main categories of leukemia:
- Acute lymphoblastic leukemia (ALL). ALL is the most common type of cancer in children from one to seven years of age and the most common leukemia in children from infancy to age 19, although it also affects adults. In young children, survival chances are especially good. In ALL, leukemic blast cells accumulate in the bone marrow and blood and the cancer progresses quickly without treatment. It can spread to the lymph nodes and central nervous system. Untreated ALL can lead to anemia, poor immunity, and easy bleeding and bruising.
- Chronic lymphocytic leukemia (CLL). This is the most common leukemia of adults in Western countries and it almost never affects teens or children. It tends to be found in those over the age of 55 or 60 and the mean age at diagnosis is 65. This disease progresses more slowly compared with some other types of leukemia. The slow-growing CLL cells don't typically block the production of normal cells in the marrow as much as they do in ALL. That is why the early stages of CLL are often not as severe as early stages of ALL. The slow-growing form of CLL can remain stable for years and does not require treatment. However, there is a faster-growing form of CLL that blocks normal blood cell production and requires treatment. People with CLL may have enlarged lymph nodes, immunoglobulin deficiencies that lead to poor immunity, autoimmunity (such as autoimmune hemolysis), and an enlarged spleen.
- Acute myeloid leukemia (AML). This is the most common acute leukemia in adults. Adults older than 50 are more likely than children to develop AML and adolescents over 15 are more likely to develop it than younger children. AML is generally a rapidly developing cancer where immature myeloid cells continually divide in the bone marrow or other tissue and can replace bone marrow with immature, dysfunctional white blood cells. Untreated AML leads to anemia, poor immunity, and very easy bruising and bleeding.
- Chronic myelogenous (myeloid) leukemia (CML). CML usually occurs in adults, with people 65 and over at greater risk. It rarely affects children. People with CML often have no symptoms at first and are often diagnosed during a routine blood test or physical. When symptoms do appear, they are similar to common, less serious illness and include low energy, pale skin, stomach discomfort caused by an enlarged spleen, and unexplained weight loss. CML can be traced to abnormal chromosomes where, inside a stem cell in the bone marrow, pieces from two chromosomes break off and switch places (translocation). This results in an altered, fused gene (called BCR/ABL) on chromosome 22 (also known as the Philadelphia chromosome). This altered gene makes an abnormally functional protein that leads to the overproduction of white blood cells. Left untreated, CML leads to anemia, poor immunity, excessive bruising and bleeding, and markedly enlarged spleen.