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What is leukemia?

Leukemia is cancer that starts in the blood-forming cells of the bone marrow. When one of these 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 the 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 the blood. Once in the blood, leukemia cells can spread to other organs, where they can prevent other cells in the body from functioning normally.

Leukemia is different from other cancers that start in organs like the lungs, colon, or breast and then spread to the bone marrow. Cancers that start somewhere else and then spread to the bone marrow are not leukemia.

Knowing the exact type of leukemia a person has helps doctors better predict their outlook and select the best treatment.

What is a chronic leukemia?

In a chronic leukemia, the leukemia cells can mature partly (and are more like normal white blood cells than the cells of acute leukemias), but they don't mature completely. 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 survive longer than normal cells and build up, crowding out normal cells in the bone marrow.

It can take a long time before chronic leukemias cause problems, and most people can live with them 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.

Lymphocytic leukemias (also known as lymphoid or lymphoblastic leukemias) start in the cells that would normally become lymphocytes. Lymphomas are also cancers that start in those cells. The main difference between lymphocytic leukemias and lymphomas is that in leukemia, the cancer cells are mainly in the bone marrow and blood; in lymphoma they tend to be in lymph nodes and other tissues.

Not all chronic lymphocytic leukemia (CLL) is the same

There seem to be different types of 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 a person has. The tests look for changes in the leukemia cells - either a deletion of part of chromosome 17 or a mutation in the TP53 gene. (The TP53 gene also is on chromosome 17, so both tests are actually looking for changes in this gene, which normally helps keep cell growth under control.) If the CLL cells have either of these changes, the leukemia tends to grow faster and might be harder to treat with certain types of medicines, so this might affect a person's treatment options.

Other gene and chromosome changes inside the CLL cells can also be important. To learn more, see How Is Chronic Lymphocytic Leukemia Staged?

Rare forms of lymphocytic leukemia

The common form of CLL starts in B lymphocytes. But there are some rare types of 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 will respond to some form of treatment, but over time the leukemia tends to come back.

PLL may develop in someone who already has CLL (in which case it tends to be more aggressive), but it can also occur in people who have never had CLL.

Large granular lymphocyte (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 fine 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.