Cancer starts when cells in the body change (mutate) and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them, and die when your body doesn't need them any more. Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most types of cancer, the abnormal cells grow to form a lump or mass called a tumor.

Leukemia is different from most other types of cancer. Leukemia is a blood cancer that starts in the bone marrow. This is where new blood cells are made. Leukemia cells are early (immature) forms of blood cells, most often white blood cells. When a person has leukemia, the body makes too many abnormal blood cells. These cells don't usually form tumors, but they can travel throughout your body in your blood. That means leukemia can affect organs all over your body.

There are 2 types of white blood cells can turn into leukemia:

• Lymphoid cells (lymphocytes). This is called lymphocytic leukemia.
• Myeloid cells (myelocytes). This is called myeloid or myelogenous leukemia.

Leukemia can also be either acute or chronic. Acute leukemia tends to grow very quickly and needs to be treated right away. Chronic leukemia often grows more slowly. Some people don't need to start treatment until years after they're diagnosed.

Chronic lymphocytic leukemia (CLL) is the most common chronic leukemia in adults. It is a type of leukemia that starts in the lymphoid cells in the bone marrow. These are cells that normally help the body fight infection. As the CLL cells grow and multiply, they can crowd out the normal cells in the bone marrow. This can lead to the body not having enough of the different types of blood cells it needs.

People with CLL have too many lymphocytes in their blood. But these cells are not normal and don't help fight infection. In fact, people with CLL are more likely to get an infection.

CLL is a type of chronic leukemia. This means it tends to grow slowly. Many people with CLL don't have any symptoms when it's first found. This kind of leukemia often doesn't need to be treated right away. But some types of CLL grow faster than others. CLL cells are tested in a lab to learn details about the type of CLL a person has. The cells needed for testing can come from the blood, or the bone marrow, or both.

If you have questions about CLL, talk with your doctor. Your doctor can help you understand more about this type of leukemia.