Leukemia is a type of cancer that results in the body making too many abnormal white blood cells. This uncontrolled production results in an excessive amount of white blood cells that may be immature (acute leukemia) or mature (chronic leukemia). The leukemic cells may not function well to fight infection and may interfere with the production of red blood cells (which carry oxygen) and platelets (which control bleeding).

Normally, white blood cells play an important role in the body's natural defence system. They target and destroy foreign invaders like viruses and bacteria. White blood cells are made in the marrow (the spongy core) of your bones. Without healthy and functioning white blood cells, the body is at risk of developing severe and sometimes fatal infections.

For most people with leukemia, there's no way to identify what causes it. In some cases, though, specific risk factors can be identified:

• previous chemotherapy or radiation therapy
• exposure to high doses of radiation or to benzene (found in unleaded gasoline, tobacco smoke, chemical production facilities)
• family history
• genetic abnormality, such as an abnormality on chromosome 22 (also known as the Philadelphia chromosome)
• genetic disorders, such as Down syndrome and Fanconi anemia

All forms of cancer that can spread within the body (malignant), including leukemia, are thought to be due to genetic abnormalities (mutations). In leukemia, the damage occurs in the bone marrow stem cells. These special cells help to manufacture all the other cells in the blood. With this condition the production of these cells is out of control.

Different types of leukemia

Acute leukemia develops within days to weeks, and large numbers of immature cells called "blasts" build up. These cells can't function as well as normal white blood cells, so people with acute leukemia are at a higher risk of infection. Because the body is so busy producing "blasts," it can't make as many red blood cells or platelets, which can cause anemia and bleeding disorders.

Chronic leukemia, which progresses over the course of months to years, involves overproduction of mature white blood cells that cannot function like normal white blood cells.

There are 4 types of leukemia:

• Acute lymphocytic leukemia (ALL) is the most common form of leukemia in children. It is the result of an uncontrolled production of a type of white blood cell called lymphocytes. This causes a buildup of the immature forms of lymphocytes. The high numbers also interfere with the production of red blood cells and platelets.
• Chronic lymphocytic leukemia (CLL) most often occurs in people over 55 years of age. It is the most common type of leukemia overall, and occurs about twice as often in men as in women. It develops at a slower rate than ALL. Gradually, leukemic cells outnumber the normal-functioning cells in certain tissues in the body, including the bone marrow where other blood cells are made.
• Acute myeloid leukemia (AML) causes uncontrolled production of another type of white blood cell called myelocytes. This causes an overgrowth of their immature cells called myeloblasts. This interferes with the levels of functioning red blood cells, platelets, and normal white blood cells. AML is the most common form of acute leukemia in adults.
• Chronic myelogenous leukemia (CML) occurs more slowly than AML and has less effect on the development of other cell types. Chances of getting CML are very low for children but increase with age. CML is associated with the Philadelphia chromosome.

Leukemia either causes a shortage of functioning white blood cells, which can impair the immune system, or a buildup of extra white blood cells.

In acute leukemias, the extra white blood cells are mostly immature, whereas in chronic leukemias the white blood cells are mature but still function poorly. In either case, fewer platelets (the type of blood cells that help stop bleeding) are produced, causing excessive bleeding. Minor injuries can result in hard-to-stop bleeding and large bruises. Tiny red spots may appear just under the skin.

Because the immune system isn't working normally, infections are more frequent. Lymph glands and the spleen may become enlarged, causing pain in the left side of the abdomen.

If abnormal cells accumulate in the brain and spinal cord (as in AML), the symptoms include vomiting and headaches. Occasionally, abnormal cells can accumulate in the skin, causing lumps (chloromas) or skin rashes. A lack of red blood cells causes the skin to look pale. It also results in fatigue and shortness of breath.

Blood and bone marrow are sampled to check the types and numbers of blood cells present. High levels of immature white blood cells (and low red blood cell and platelet counts) indicate acute leukemia.

Special tests such as cytogenetic analysis and flow cytometry can help identify the abnormal cells. Knowing the specific type of leukemia helps the doctor determine the appropriate treatment.

CLL can result in an elevated white blood cell count that can remain stable for years and not require treatment. Often, though, mild treatments are required to keep the total count within a manageable range. In rare cases, more aggressive treatments are needed for changes in the white blood cell count or to treat complications such as anemia.

Doctors commonly treat CML with a class of medication called tyrosine kinase inhibitors. This class is also called targeted molecular therapies and works by interfering with the abnormal cells' ability to overstimulate production of various types of blood cells. After the treatment, many people with leukemia have no signs of leukemia (also called remission) for short periods of time, and some go into long-term remissions in which abnormal cells are no longer found in their blood.

White blood counts and overall health must be monitored, but sufferers can usually continue with normal activities during the therapy. More aggressive treatments are used to try to get acute leukemias into remission – the stage where normal levels of blood cells are restored. A combination of different chemotherapy medications is used for this. Some people may require radiation and a stem cell transplantation.

Bone marrow transplants combined with chemotherapy for people with CML may result in a cure, but it is usually reserved for people who do not respond to other therapies. Most people with CML start with tyrosine kinase inhibitors that act on the Philadelphia chromosome, because of their success with the ability to induce a remission.