Chronic lymphocytic leukemia (CLL) is a type of blood cancer that begins in the bone marrow. It affects white blood cells called lymphocytes. These white blood cells are part of the immune system and help your body fight infection.

CLL is the most common type of leukemia in adults. This kind of leukemia is considered chronic, because it usually develops slowly over a long period of time.

Most people with CLL don’t have any signs or symptom for years. But, in some cases, the disease can be aggressive and faster-growing.

Many people with CLL don’t have any symptoms when they’re diagnosed.

This type of leukemia may be found when a doctor orders blood tests for a routine check-up or an unrelated health condition. If a high number of lymphocytes show up on a blood test, it might lead your doctor to suspect leukemia.

For the small number of people who do experience symptoms early in the course CLL, the symptoms may be subtle or confused for another more common illness.

These symptoms include:

Another early symptom of CLL that may be noticed before other symptoms is a painless swelling of the lymph nodes. Common areas of swollen lymph nodes are the underarms and groin.

Some people may also experience pain or fullness in the stomach. This happens when CLL has caused the spleen to become enlarged.

Later-stage or faster-developing CLL may cause more serious or obvious signs and symptoms in some people.

These include:

  • Frequent infections. Later-stage CLL may cause chronic upper and lower respiratory tract infections. This may happen because this kind of leukemia can trigger leukopenia or a shortage of the white blood cells that fight infection. You may also get skin infections. More severe infections of the lungs, kidneys, and other organs can also develop.
  • Severe anemia. More serious kinds of anemia, such as autoimmune hemolytic anemia, can happen in 7 to 10 percent of all people with CLL. This happens when the leukemia forms abnormal antibodies that attack red blood cells (RBCs), leading to low levels of oxygen-carrying RBCs in the body.
  • Severe or chronic fatigue. Because CLL can cause anemia (a shortage of red blood cells) and low oxygen levels, this leads to ongoing or severe fatigue.
  • Easy or abnormal bruising or bleeding. CLL can cause a condition called thrombocytopenia (reduced blood platelets) in 2 to 5 percent of people with this type of leukemia. Lower levels of platelets affect how well your blood clots. This can cause more or abnormal bleeding and bruising, such as bleeding gums or nosebleeds.
  • Headache and other neurological symptoms. In rare and more advanced cases, CLL can affect the central nervous system. This can cause brain and nervous symptoms, like:
  • Other cancers. Some people with later-stage CLL have a higher risk of developing other kinds of cancers. These include skin cancer, lung cancer, and cancers of the digestive tract. A very small number of people with CLL may develop a more aggressive form of cancer called diffuse large B-cell lymphoma or Richter’s syndrome.

The exact cause of CLL is not known. What is known is that gene changes may happen in the cells that make blood cells. This mutation triggers blood cells to make abnormal white blood cells (lymphocytes), which aren’t effective at fighting infection.

The abnormal lymphocytes multiply and accumulate in the blood and other organs, like the liver and spleen, where they cause complications.

Some factors may raise your risk of developing CLL:

  • Family history. If your first-degree relative (parent, sibling, or child) has had CLL, you have a 5 to 7 times greater chance of developing it as well.
  • Age. CLL is most common in older adults and risk increases with age. CLL rarely affects children and is rare under age 30 The average age at diagnosis is around 72 years.
  • Gender. Men are twice as likely to develop CLL than women.
  • Ethnicity. A type of CLL called B-cell CLL is more common in people of Russian and European descent. It also commonly occurs in African American and Black people. The reason for these differences is not known.
  • Toxin exposure. Some toxic environmental chemicals may raise the risk of CLL in some people. The U.S. Department of Veterans Affairs lists CLL as a disease linked to exposure to Agent Orange, a chemical used during the Vietnam War.

CLL is a cancer that affects white blood cells called lymphocytes and happens most often in older adults.

Many people might not realize they have CLL, because they might not have any symptoms in the early stages, or the symptoms are very mild.

In others, CLL may advance faster and cause more severe symptoms, as well as other complications.

Early stage CLL may take years to progress or happen sooner for some people. Let your doctor know if you experience any symptoms that are unusual for you or that last longer than normal.