Worried about Coronavirus? Visit our Coronavirus hub for the latest information and advice.

If you’ve got CLL, your body produces too many of a certain type of white blood cell called lymphocytes which don’t work properly. CLL is a chronic condition, which means it usually develops very slowly.

CLL is the most common leukaemia in adults in the UK with around one person in 200 people developing CLL at some point in their life.

What is chronic lymphocytic leukaemia?

If you have CLL, your body produces too many lymphocytes (a type of white blood cell) that don’t work properly. These cells won’t fight infection as well as normal white blood cells do, and they multiply in an uncontrolled way and stop normal blood cells from working properly. This is what causes most of the symptoms that people with CLL get.

What causes CLL?

When you’re diagnosed with any cancer, one of the first things you might think is: why me?

With CLL, there are no clear reasons other than things like age and gender. Here’s what we do know:

  • CLL is the most common type of leukaemia in adults.
  • Of all people diagnosed with leukaemia, around a third are diagnosed with CLL.
  • About one person in 200 will develop CLL at some point in their life.


People who get CLL are usually in their 70s or older. It’s very rare for people under 40 to get CLL, but about 10% of patients are under 55 when diagnosed. Children don’t get CLL.


Men are about twice as likely as women to get it – we don’t know why.

Family history

A parent, child, brother or sister of someone with CLL has a slightly higher chance than others of developing the condition, but the risk for any individual is still very low.

Because of this, experts don’t recommend testing family members unless they’re being considered as donors for a stem cell transplant – which is rare for people with CLL.


CLL is more common in white people – we don’t know why.

Monoclonal B-cell lymphocytosis

There’s a condition called monoclonal B-cell lymphocytosis (MBL) where people have very low levels of CLL-like cells in their blood, but no symptoms.

Not all people with MBL go on to get CLL. However, we think that most people diagnosed with CLL previously had MBL, possibly for many years.

Experts don’t recommend testing family members for MBL unless they’re being considered as donors for a stem cell transplant.

Radiation and chemicals

There have been some suggestions that exposure to radiation or certain chemicals may increase your risk of developing CLL, but this link hasn’t been proven.

Your immune system

Your immune system is a network of cells, tissues and organs which protect your body against infection. It’s able to react quickly to infections it’s seen before, and lymphocytes play an important role in this. They circulate around your body in your blood and fight infections. There are lots of different kinds of lymphocyte, but the important ones to know are T cells and B cells. 

CLL affects B cells, which normally produce antibodies to fight infections. Because of this, CLL often affects your body’s defences. This means you might get:


They could be more severe than usual and last longer. Your healthcare team can let you know ways to reduce your risk of infections.

Auto-immune conditions

Normally your immune system knows your own cells and won’t harm them. However, if you’ve got CLL, your immune system can get confused and damage some of your tissues. Anaemia, low platelet count and rheumatoid arthritis are all examples of auto-immune conditions.


Shingles is an infection of a nerve and the skin around it. It can affect you if you’ve had chickenpox, even if you had it a long time ago.

You’re more likely to get shingles if your immune system isn’t working well – for example, if you have CLL.

Your lymphatic system

A network of thin tubes called lymph vessels runs around your body. This is your lymphatic system. The vessels collect fluid called lymph and return it to your blood.

Along the lymph vessels are small lumps of tissue called lymph nodes or lymph glands. There are many of these in your body. If you get an infection when you’re healthy, these can swell and become tender – people usually call them 'swollen glands'.

Although CLL mainly affects the bone marrow, it often causes swelling in lymph nodes because abnormal lymphocytes build up in them. The ones in your neck, armpits and groin are often affected but, more rarely, those in your gut lining can be too. CLL can also cause swelling in your spleen, the most important organ in your immune system, meaning that it can’t work properly.

If CLL is affecting your lymphoid tissues a lot, it may be called small lymphocytic lymphoma (SLL), which is a type of CLL. Your treatment will still be the same though and your condition won’t develop any differently.

Information and resources on CLL

As the information isn't produced by Bloodwise, we can't guarantee the content of these pages. We've marked information that is more suitable for healthcare professionals.

Blood and bone marrow

Blood components from NHS Blood and Transplant

The immune system

What is immunology? from the British Society for Immunology


CLL statistics - incidence from Cancer Research UK (CancerStats)

QuickStats on incidence of blood cancers from the Haematological Malignancy Research Network


CLL – general discussion of CLL on Patient [for healthcare professionals]

CLL risks and causes from Cancer Research UK

Signs and symptoms

Symptoms of CLL from Cancer Research UK

Tests and investigations

Lab tests online - Department of Health approved website with details on tests and investigations


Treatment of CLL from CLL Support Association

Treatment decisions for CLL from Cancer Research UK

How I treat CLL up front from Blood journal [for healthcare professionals]


Follow up for CLL by Cancer Research UK

Conditions related to CLL

Hairy cell leukaemia from CLL Support Association

Patient Information Forum member NHS Information Standard certified member