Chronic lymphocytic leukemia (CLL) is a type of cancer that starts in the white blood cells (lymphocytes) in the bone marrow. It then invades the blood. Leukemia always starts in the bone marrow. It can spread to the lymph nodes, the spleen, liver, and other parts of the body. In contrast, other types of cancer can start in these organs and then spread to the bone marrow (or elsewhere). Those cancers are not leukemia. Leukemia cells tend to build up in the body over time. In many cases people don't have any symptoms for at least a few years. Compared to other types of leukemia, CLL usually grows slowly.
Doctors have learned that there are probably 2 different kinds of CLL.
One kind is very slow growing and rarely needs to be treated. People with this kind of CLL survive an average of 15 years or more.
Another kind grows faster and is more serious. People with this form of CLL have an average survival of about 8 years.
The leukemia cells from these 2 types look alike. But certain new lab tests can tell them apart.
Normal Bone Marrow, Blood, and Lymph Tissue
In order to understand the different types of leukemia, it helps to have some basic knowledge about the blood and lymph systems.
Bone Marrow
Bone marrow is the soft inner part of some bones such as bones of the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made up of blood-forming cells, fat cells, and tissues that help cells grow.
Early blood cells are called stem cells. These stem cells only make new blood-forming cells. They cannot make any other kinds of cells. (This makes them different from embryonic stem cells, which are formed from a developing fetus and can develop into most other cells in the body.)
Stem cells normally divide to form new stem cells. But some of the stem cells go through a series of changes. During this process they can become red blood cells, white blood cells, or platelets.
Red Blood Cells
Red blood cells carry oxygen from the lungs to all other cells in the body. They also carry away carbon dioxide, a waste product of cell activity. A shortage of red blood cells (anemia) causes weakness, shortness of breath, and tiredness.
Platelets
Platelets help prevent bleeding by plugging up holes of blood vessels caused by cuts or bruises. A person with a shortage of platelets can bruise or bleed easily.
White Blood Cells
White blood cells help defend the body against infections. Lymphocytes are one type of white blood cell, but there are other types too. Lymphocytes are the main cells that make up lymphoid tissue, an important part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.
There are two types of lymphocytes:
B lymphocytes protect the body from invading germs. B lymphocytes are the cells that most often develop into chronic lymphocytic leukemia (CLL) cells.
T lymphocytes destroy cells that are infected with viruses.
How Leukemia Starts
Any of the blood-forming or lymphoid cells from the bone marrow can turn into leukemia cells. Once this happens, the cells don’t go through their normal growth process. These cells do not die as they should. So they build up, spill into the bloodstream, and spread to other organs.
Types of Leukemia
Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the exact type of leukemia can help doctors better predict each patient's outlook (prognosis) and select the best treatment.
Acute Leukemia Versus Chronic Leukemia
In acute leukemia, the cells grow rapidly but they are not able to mature properly. The cells keep on reproducing and build up in the blood. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured.
Chronic leukemia refers to a condition where the cells look mature but they are not really normal. The cells live too long, build up, and crowd out normal cells.
Myeloid Leukemia Versus Lymphocytic Leukemia
The second factor to consider in classifying leukemia is the type of bone marrow cells that are involved. Lymphocytic and myeloid (or myelogenous) refer to these cell types involved.
Myeloid leukemias start from any other type of white blood cell, red blood cell, or platelet-making cell in the bone marrow.
Lymphocytic leukemia: Lymphocytic leukemias develop from lymphocytes (a type of white blood cell) in the bone marrow.
What Causes Chronic Lymphocytic Leukemia? Can It Be Prevented?
A risk factor is something that increases a person's chance of getting a disease. Some risk factors, like smoking, can be controlled. Others, such as a person's age, can't be changed. There are very few known risk factors for chronic lymphocytic leukemia (CLL).
Certain Chemical Exposures
Exposure to Agent Orange, an herbicide used during the Vietnam War, has been linked to an increased risk of CLL. Some studies suggest that chemicals used in farming, such as herbicides and insecticides, may increase the risk of CLL. More research in this area is needed.
Family History
First-degree relatives (parents, siblings, or children) of CLL patients have an increased risk for this cancer.
Gender
CLL is slightly more common in men than women, although the reasons for this are not known.
Race/Ethnicity
CLL is more common in North America and Europe than in Asia.
There are no other proven risk factors for CLL. The risk of getting CLL does not seem to be affected by smoking, diet, radiation, or infections.
Can Chronic Lymphocytic Leukemia Be Prevented?
Although many types of cancer can be prevented by lifestyle changes to avoid certain risk factors, there are no known risk factors for CLL that a person can change. So right now there is no way to prevent CLL from developing.
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