Leukaemia is a cancer of the white blood cells. White blood cells are produced by the bone marrow. The four main types of leukaemia are chronic lymphocytic (CLL), acute myeloid (AML), acute lymphoblastic (ALL) and chronic myeloid (CML). Each type of leukaemia has its own characteristics and treatment.

The bone marrow

Bone marrow is a spongy material that fills the middle of some bones and produces cells called stem cells. These are immature cells that develop into the three different types of blood cells:

• red blood cells, which carry oxygen to all the cells in the body
• white blood cells, which are essential for fighting infection
• platelets, which help the blood to clot and so control bleeding.

All these cells normally stay inside the bone marrow until they are fully developed (mature). They are then released into the blood to circulate around the body and perform their functions.

White blood cells

The bone marrow produces two main types of white blood cells: granulocytes and lymphocytes. These work together to fight infection in the body.

Lymphocytes can also pass into the lymphatic system, which is part of the body's natural defence against infection. It is made up of a network of small lymph nodes (sometimes called glands) throughout the body connected by very fine lymphatic vessels. These vessels contain a milky looking fluid called lymph which contains lymphocytes.

Lymph nodes are found mainly in the neck, armpit and groin. There are also some lymph nodes in the chest and the abdomen. The tonsils, spleen (which breaks down old blood cells) and bone marrow are part of the lymphatic system and contain lymphocytes. The spleen is under the ribs on the left hand side of the abdomen.