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Chronic lymphocytic leukemia (CLL) is one of the most common types of leukemia in adults. It's a type of cancer that starts in early forms of certain white blood cells (called lymphocytes) in the bone marrow. The cancer (leukemia) cells start in the bone marrow but then go into the blood.

In CLL, the leukemia cells often build up slowly. Many people with CLL don't have any symptoms for at least a few years. But over time, the cells can build up and spread to other parts of the body including the lymph nodes, liver, and spleen.

What is leukemia?

Leukemia is cancer that starts in the blood-forming cells of your bone marrow. When one of these blood-forming cells changes and becomes a leukemia cell, it no longer matures the way it should and grows out of control. Often, it divides to make new cells faster than normal.

Leukemia cells also don't die when they should. This allows them to build up in your bone marrow, crowding out normal cells.

At some point, leukemia cells leave the bone marrow and spill into the bloodstream. This increases the number of white blood cells in your blood. Once leukemia cells are in your blood, they can spread to other organs, where they can prevent other cells in your body from working normally.

Knowing the exact type of leukemia you have will help your cancer care team better predict your outlook and select the best treatment.

What is a chronic leukemia?

Leukemia is either chronic or acute. Chronic leukemia is slower growing. Acute leukemia grows more quickly.

CLL is a chronic (slower growing) leukemia.

In a chronic leukemia, the leukemia cells can mature partly, but they don't mature completely. They are more like normal white blood cells than the cells of acute leukemias. These cells may look fairly normal, but they're not. They generally don't fight infection as well as normal white blood cells do.

The leukemia cells typically survive longer than normal cells. They build up over time, crowding out normal cells in the bone marrow.

It can take a long time before chronic leukemia causes problems. Most people can live with it for many years. But chronic leukemias tend to be harder to cure than acute leukemias.

What is a lymphocytic leukemia?

Leukemia is either myeloid or lymphocytic, depending on which bone marrow cells the cancer starts in.

CLL is a lymphocytic leukemia.

Lymphocytic leukemias (also known as lymphoid or lymphoblastic leukemias) start in the bone marrow cells that would normally become white blood cells called lymphocytes.

Lymphomas are also cancers that start in early forms of lymphocytes. The main difference between lymphocytic leukemia and lymphoma is that in leukemia, the cancer cells are mainly in the bone marrow and blood. In lymphoma, the cancer cells tend to be in lymph nodes and other body tissues.

Types of chronic lymphocytic leukemia (CLL)

There are different types of chronic lymphocytic leukemia (CLL), based on some of the gene changes inside the cells:

• One type of CLL tends to grow very slowly, so it may be a long time before a person needs treatment.
• Another type of CLL tends to grow faster and doesn't respond as well to certain kinds of treatments.

Lab tests can be done on the leukemia cells to tell which type of CLL you have. These tests look for changes in the leukemia cells.

If the CLL cells have  either a deletion of part of chromosome 17 (written as del(17p)) or a mutation in the TP53 gene, the leukemia tends to grow faster and might be harder to treat with certain types of medicines. This might affect your treatment options.

The TP53 gene is on chromosome 17, so both tests are actually looking for changes in this gene, which normally helps keep cell growth under control.

Other gene and chromosome changes inside the CLL cells can also be important, such as having an extra copy of chromosome 12 (trisomy 12).

To learn more, see Chronic Lymphocytic Leukemia (CLL) Stages.

Small lymphocytic lymphoma (SLL)

CLL and small lymphocytic lymphoma (SLL) can be thought of as different versions of the same disease, so much so that they are often grouped together (as CLL/SLL).

The cancer cells of SLL and CLL look the same under the microscope. They also have the same gene mutations and the same marker proteins on the surface of the cells.

The main difference between SLL and CLL is the location of most of the cancer cells in the body:

• CLL is mainly in the blood and bone marrow.
• SLL is mainly in the lymph nodes and spleen (and sometimes other organs).

SLL and CLL are treated in basically the same way.

Rare forms of lymphocytic leukemia

The common form of CLL starts in B lymphocytes. But there are a few rare types of lymphocytic leukemia that share some features with CLL.

Prolymphocytic leukemia (PLL)

In this type of leukemia, the cancer cells are a lot like normal cells called prolymphocytes. These are immature forms of either B lymphocytes (B-PLL) or T lymphocytes (T-PLL).

Both B-PLL and T-PLL tend to grow and spread faster than the usual type of CLL. They often respond to some form of treatment, but over time the leukemia tends to come back.

PLL may develop in someone who already has CLL. When this happens, it tends to be more aggressive. But it can also develop in people who have never had CLL.

Large granular lymphocytic (LGL) leukemia

In this rare form of chronic leukemia, the cancer cells are large and have features of either T lymphocytes or another type of lymphocyte called natural killer (NK) cells.

Most LGL leukemias are slow growing, but a small number can grow and spread quickly. Drugs that suppress the immune system may help, but the aggressive types of leukemia can be hard to treat.

Hairy cell leukemia (HCL)

This type of leukemia gets its name from the way the cells look under the microscope. They have thin projections on their surface that make them look "hairy."

This is a rare cancer of the lymphocytes that tends to progress slowly. The cancer cells are a type of B lymphocyte, but they're different from those seen in CLL. There are also important differences in symptoms and treatment.

Treatment for HCL is usually successful.