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What is chronic lymphocytic leukaemia (CLL)?

Chronic lymphocytic leukaemia (CLL) is a type of cancer that affects the blood and bone marrow. It affects the white blood cells called lymphocytes. It tends to develop very slowly.

It is more common in older people and is rare in people younger than 40. 

What is leukaemia?

Leukaemia is a blood cancer. Blood cells are made in your bone marrow. The bone marrow is the soft inner part of some of your bones.

In most types of leukaemia, abnormal white blood cells are made in the bone marrow. These cells can get into the bloodstream and circulate round the body. They do not develop properly and so do not work normally. For example, they don't give you the protection from infection that they should. There are too many of these abnormal white blood cells. They might stop the bone marrow producing enough healthy blood cells.

They can also build up in the lymph nodes and spleen and cause swelling. 

There are different types of leukaemia. They are divided into two main groups:

  • acute leukaemia
  • chronic leukaemia

Acute and chronic leukaemia

Acute leukaemias tend to develop quickly and get rapidly worse if they are not treated. Chronic leukaemias develop slowly and tend to get worse slowly, over a long time. 

In chronic leukaemia the white blood cells are almost fully developed, but are not completely normal. They still work, but not as well as they should do at fighting infection. Your body makes too many of these abnormal white blood cells. 

Blood cells and leukaemia

To understand how and why leukaemia affects you as it does, it helps to know more about blood cells.

All blood cells start as the same type of cell, called a stem cell.

This stem cell then develops into:

  • myeloid stem cells, which become white blood cells called monocytes and neutrophils (granulocyte), red blood cells and platelets
  • lymphoid stem cells, which become white blood cells called lymphocytes

The diagram below helps to explain this.

Diagram showing how blood cells are made

The type of chronic leukaemia you have tells you which type of white blood cell has become cancerous. In chronic lymphocytic leukaemia (CLL), it is the lymphocyte white blood cells that are cancerous.

Diagram showing Blood cells and leukaemia

How leukaemia affects the blood cells

White blood cells help to fight infection. If you have abnormal white blood cells they cannot fight infection so well. You may get a lot of infections, which may be difficult to get rid of.

When too many white blood cells are made, they take up much more room in the bone marrow than they would normally. This means that there is not enough space for making normal red blood cells and platelets. Red blood cells carry oxygen round the body. If you don't have enough of these, you have anaemia. This can make you tired and breathless.

Platelets are vital for normal blood clotting. If you do not have enough platelets, you will have bleeding problems such as nosebleeds, very heavy periods or a fine rash of red spots caused by bleeding into the skin (petechiae).

Blood cells normally die when they are worn out. In some types of chronic leukaemia the blood cells are not over produced but they don't die when they are worn out. So, there are too many white blood cells than is normal.

Chronic leukaemia

The two most common types of chronic leukaemia are:

  • chronic myeloid leukaemia (CML)
  • chronic lymphocytic leukaemia (CLL)

The difference is the type of white blood cell that has become cancerous.

In CLL the abnormal cells develop from the lymphoid blood stem cells. The cancerous white blood cells are B lymphocytes, also called B cells. Lymphocytic in CLL is pronounced lim-fo-sit-ik.

In CML, the abnormal cells develop from the myeloid blood stem cells. So the cancerous white blood cells are myelocytes. These cells are sometimes called granulocytes.

You may hear this type of leukaemia called chronic granulocytic leukaemia (CGL). Myeloid is pronounced my-el-oyd and granulocytic is pronounced gran-you-low-sit-ik.

If you are looking for information about chronic myeloid leukaemia, this is not the right section for you.

Other types of chronic leukaemia

Rarely, CLL can change over time into another type of leukaemia called prolymphocytic leukaemia. Doctors call this transformation. Sometimes prolymphocytic leukaemia is diagnosed in people who have not had CLL.

Advanced CLL can sometimes develop into a cancer of the lymphatic system (a lymphoma). This is called Richter's syndrome. Between 2 and 10 out of every 100 people (2-10%) with CLL develop Richter's syndrome.

Hairy cell leukaemia is a type of chronic leukaemia that is rarer than CLL or CML. The leukaemia cells have outgrowths that look like tiny hairs on their surfaces. These can be seen under a microscope and give this type of leukaemia its name.

How common is chronic lymphocytic leukaemia?

Around 3,800 people are diagnosed with chronic lymphocytic leukaemia in the UK each year. CLL is by far the most common type of chronic (slowly developing) leukaemia.

CLL is more common in older people and is rare in people under 40. Men are more likely to develop CLL than women. We don't know why that is.

Although leukaemia is the most common type of childhood cancer, leukaemia in children is nearly always acute leukaemia – either acute myeloid leukaemia or acute lymphoblastic leukaemia. Chronic leukaemia is very rare in children.

Last reviewed: 
29 Sep 2020
Next review due: 
29 Sep 2023
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