What is CLL?

Chronic Lymphocytic Leukaemia (CLL) is a type of blood cancer that occurs when your body makes too many abnormal white blood cells.

Chronic Lymphocytic Leukaemia (CLL) is a type of blood cancer that occurs when your body makes too many abnormal white blood cells. Under normal conditions, healthy white blood cells help our bodies fight infection and disease. Leukaemia develops when cancerous white blood cells develop in the bone marrow and invade the circulating blood, eventually outnumbering the normal-functioning cells.

In CLL, the white blood cells which become cancerous are called lymphocytes. Their function is to recognise bacteria, viruses and toxins, produce antibodies to them, and destroy them. As well as being in the blood and bone marrow, white blood cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues. CLL can behave very differently in different people. The term ‘chronic’ means that in most cases this type of cancer is ongoing and develops, CLL Advocates NZ or progresses slowly (if at all) over months and years, even without treatment. However, in some cases the disease progresses more
rapidly and may need early treatment. To understand CLL it is helpful to know about the different types of
blood cells and how they are made.

How are blood cells made?

Blood cells are produced inside the bone marrow, the sponge-like material found in the centre of the bones. Production of new blood cells is very closely controlled to balance the loss of worn-out cells or cells lost by bleeding or damage. To sustain the necessary levels of blood cells the bone marrow of an adult must produce more than three million blood cells every second. The healthy number of different types of blood cells varies between people but is usually kept within fairly narrow limits.

All mature blood cells originate from immature blood cells called haematopoietic stem cells. In Greek, the word haemato means ‘blood’ and the word poietic means ‘to make’. Less than one in 5,000 bone marrow cells is a stem cell. Haematopoietic stem cells may divide to become a myeloid producing cell (common myeloid progenitor) or a lymphoid producing cell (common lymphoid progenitor). These immature blood cells go through several stages of development to make the different types of mature blood cells which are then released from the bone marrow into the blood stream where they carry out different functions.

The common myeloid progenitor cell matures into one of three different types of blood cell:

  • Red blood cells (erythrocytes) that carry oxygen and other substances to all tissues of the body.
  • Platelets (thrombocytes) that form blood clots to stop bleeding.
  • Five types of white blood cells (leukocytes): mast cells, basophils, neutrophils, eosinophils and monocytes that form part of the immune system to defend the body against infection and disease.

The lymphoid common progenitor cell matures into another type of white blood cell called a lymphocyte. There are three different types of lymphocyte:

  • B-lymphocyte (or B-cell) that makes antibodies to help fight infection.
  • T-lymphocyte (or T-cell) that helps B-lymphocytes make antibodies to help fight infection. They also help kill viruses, fungi and cancer cells.
  • Natural killer cell that attacks cancer cells and viruses.

Since CLL is a cancer of the B-lymphocytes, it can also affect the glands of the lymphatic system (lymph nodes), the spleen and other organs. Consequently, affected CLL patients have impaired immune systems and are at greater risk of infections.

When abnormal B-cells accumulate only in the lymph nodes rather than in the blood, the cancer is referred to as small lymphocytic lymphpoma (SLL). SLL and CLL are slightly different forms of the same disease, but both conditions respond to the same form of treatment.

How common is it?

CLL is the most common form of leukaemia in adults in Western countries. Approximately 3,500 adults are diagnosed with CLL each year in the UK alone, which is equivalent to 10 new cases every day. Slightly more men than women tend to be affected by CLL and it is often diagnosed in older people, with 59% of diagnoses made in people aged 70 years and over. For reasons that are not understood, CLL is much more common in white people and less common in Asians. The disease is rarely, if ever, seen in children.