Leukemia is a hematologic cancer, meaning a cancer of the blood or blood-forming tissues, such as bone marrow. It develops as a result of genetic errors in immature blood cells, known as stem cells. The stem cells reproduce themselves over and over again, leading to an overabundance of abnormal blood cells that never mature, continue to multiply uncontrollably and enter the bloodstream.

Leukemia patients have an abnormal proliferation of blood cells, usually leukocytes (white blood cells that fight infection). White blood cells normally grow and develop in an orderly manner in bone marrow. However, in leukemia patients, their bone marrow produces large numbers of leukocytes that do not function properly.

There are two major forms of leukemia: acute and chronic. Patients with acute leukemia have a rapid increase of immature white blood cells that prevent bone marrow from producing normal blood cells. Acute leukemia should be treated immediately because these rapidly accumulating cancer cells quickly become life threatening.

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Chronic leukemia is characterized by an excessive accumulation of abnormal leukocytes that are relatively mature, resulting in a high number of abnormal white blood cells in the blood. Chronic leukemia may take months or years to progress. If you have been diagnosed with chronic leukemia, your doctor may monitor your condition for some time before treating it.

Chronic and acute forms of leukemia are further subdivided according to the types of blood cells affected, including lymphocytic (or lymphoblastic) cells and myeloid (or myelogenous) cells. These are the main subtypes of leukemia:

In addition to these more common types of leukemia, a rare type of chronic leukemia, hairy cell leukemia, is characterized by abnormal lymphocytes in bone marrow, blood and the spleen. When these cells are looked at under a microscope, they appear to be covered with tiny hairs.

Risk Factors

Leukemia researchers have identified certain risk factors for the different types of leukemia, although no one knows for sure the exact causes of this disease.

Exposure to very high levels of radiation is a risk factor for AML, CML and ALL. Scientists found that Japanese people exposed to very high levels of radiation from atomic bomb explosions during World War II were at increased risk of developing leukemia. The same was true following the nuclear power plant accident in Chernobyl, Ukraine, in 1986. A more common source of high levels of radiation is radiation therapy, which can increase the risk of leukemia.
Exposure to certain chemicals, such as benzene, pesticides or herbicides, and formaldehyde, when high levels of these chemicals are in the environment or workplace for long periods of time, may cause AML, CML or ALL. Cigarette smoke and gasoline contain benzene. Smoking cigarettes itself is a risk factor for AML.
Certain chemotherapy agents have also been associated with a small increased risk of developing AML or ALL, especially chemotherapy containing alkylating agents (drugs that interfere with cellular DNA and inhibit cancer cell growth) or topoisomerase inhibitors (drugs that block enzymes needed for cells to divide and grow).
Some medical conditions can also increase the risk of acute leukemia, such as Down syndrome and other genetic disorders that involve chromosomal abnormalities, myelodysplastic syndromes (diseases marked by abnormal blood-cell production) and infection with human T-cell leukemia virus-1.

Symptoms

Symptoms of leukemia can vary from patient to patient depending on the number of leukemia cells in the bloodstream, the type of leukemia and the area of the body where leukemia cells collect. People with chronic leukemia may have no symptoms, and the disease could be discovered with routine blood tests. However, people with acute leukemia usually feel sick. They may have vomiting, confusion, headaches, loss of muscle control, or seizures if the brain is affected. Leukemia may also affect the digestive tract, heart, lungs, kidneys or testes.

Table 1. Common Symptoms of Acute & Chronic Leukemia

Fever, chills or night sweats

Swollen or tender lymph nodes that are not painful

Frequent and recurrent infections caused by malfunctioning white blood cells

Weakness or tiredness

Easy bruising and bleeding, such as bleeding gums, purple patches on skin or tiny red spots under skin

Unexplained weight loss or loss of appetite

Discomfort or swelling in the abdomen due to a swollen spleen or liver

Bone or joint pain