Leukemia is a form of cancer that affects the body's ability to make healthy blood cells. It starts in the bone marrow, the soft center of various bones. This is where new blood cells are made. Blood cells include
Red blood cells, which carry oxygen from the lungs to the body's tissues and take carbon dioxide to the lungs to be exhaled
Platelets, which help blood clot
White blood cells, which help fight infections, viruses, and diseases.
Although cancer can affect red blood cells and platelets, leukemia generally refers to cancer of the white blood cells. The disease usually affects one of the two major types of white blood cells: lymphocytes and granulocytes. These cells circulate throughout the body to help the immune system fight off viruses, infections, and other invading organisms. Leukemias arising from lymphocytes are called lymphocytic leukemias; those from granulocytes are called myeloid, or myelogenous, leukemias.
Leukemia is either acute (comes on suddenly) or chronic (lasts a long time). Also, the type of leukemic cell determines whether it is an acute leukemia or chronic leukemia. Chronic leukemia rarely affects children; acute leukemia affects adults and children.
Leukemia accounts for about 2% of all cancers. Men are more likely to develop the disease than women, and whites are more likely to develop it than people of other racial or ethnic groups. Adults are much more likely to develop leukemia than children. In fact, leukemia occurs most often in elderly people. When the disease occurs in children, it generally happens before age 10.
Leukemia has several possible causes. These include
Exposure to radiation and chemicals such as benzene (found in unleaded gasoline) and other hydrocarbons
Exposure to agents used to cure or control other cancers, including radiation
Certain genetic abnormalities, such as Down syndrome.
Leukemia is not believed to be inherited; most cases occur in people without any family history of the disease. However, some forms of leukemia, such as chronic lymphocytic leukemia, occasionally strike close relatives in the same family. But most of the time, no specific cause can be identified.
With acute leukemia, immature white blood cells multiply quickly in the bone marrow. Over time, they crowd out healthy cells. (Patients may notice that they bleed a lot or suffer from infections as a result.) When these cells reach high numbers, they can sometimes spread to other organs, causing damage. This is especially true in acute myeloid leukemia. The two main types of acute leukemia involve different types of blood cells:
Acute lymphocytic leukemia (ALL) is the most common type of leukemia in children, mainly affecting those under age 10. Adults sometimes develop ALL, but it is rare in people older than 50. ALL occurs when primitive blood-forming cells called lymphoblasts reproduce without developing into normal blood cells. These abnormal cells crowd out healthy blood cells. They can collect in the lymph nodes and cause swelling.
Acute myeloid leukemia (AML) accounts for half of leukemia cases diagnosed in teenagers and in people in their 20s. It is the most common acute leukemia in adults. AML occurs when primitive blood-forming cells called myeloblasts reproduce without developing into normal blood cells. Immature myeloblasts crowd the bone marrow and interfere with the production of normal blood cells. This leads to anemia, a condition in which a person does not have enough red blood cells. It can also lead to bleeding and bruising (due to a lack of blood platelets, which help the blood to clot) and frequent infections (due to a lack of protective white blood cells).
Both ALL and AML have multiple subtypes. The treatment and prognosis may vary somewhat, depending on the subtype.
Chronic leukemia is when the body produces too many blood cells that are only partially developed. These cells often cannot function like mature blood cells. Chronic leukemia usually develops more slowly and is a less dramatic illness than acute leukemia. There are two main types of chronic leukemia:
Chronic lymphocytic leukemia (CLL) is rare in people under age 30. It is more likely to develop as a person ages. Most cases occur in people between ages 60 and 70. In CLL, abnormal lymphocytes cannot fight infection as well as normal cells can. These cancerous cells live in the bone marrow, blood, spleen, and lymph nodes. They can cause swelling, which appears as swollen glands. People with CLL can live a long time, even without treatment. Most often, CLL is discovered when a person has a routine blood test that shows elevated levels of lymphocytes. Over time, this type of leukemia can require treatment, especially if the person has infections or develops a high white blood cell count.
Chronic myeloid leukemia (CML) occurs most often in people between ages 25 and 60. In CML, the abnormal cells are a type of blood cell called myeloid cells. CML usually involves a defective string of DNA called the Philadelphia chromosome. (This disease is not inherited; the change in DNA that causes it occurs after birth.) The genetic defect results in the production of an abnormal protein. Drugs called tyrosine kinase inhibitors block the function of this abnormal protein, improving a person's blood counts. In some cases, the abnormal genetic defect even seems to disappear. Alternatively, some cases of CML can be cured with a bone marrow transplant.
Both CLL and CML have subtypes. They also share some characteristics with other forms of leukemia. The treatment and prognosis may vary depending on the subtype.
Rarer forms of leukemia
Lymphatic and myelogenous leukemias are the most common. However, cancers of other types of bone marrow cells can develop. For example, megakaryocytic leukemia arises from megakaryocytes, cells that form platelets. (Platelets help blood to clot.) Another rare form of leukemia is erythroleukemia. It arises from cells that that form red blood cells. Like chronic and acute leukemias, rare forms of the disease can be categorized into subtypes. The subtype depends on what markers the cells carry on their surface.