Bone marrow is a spongy material that fills some of our bones and produces stem cells. Stem cells are cells at a very early stage of development. All the blood cells in the body develop from stem cells. A type of stem cell called a myeloid stem cell can make:

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

There are different types of white blood cell. The two main types are granulocytes and lymphocytes and these work together to fight infection.

Blood cells go through various stages of development before they are fully mature and able to work properly. Normally, while blood cells are developing, they stay inside the bone marrow. Once they are mature, they leave the bone marrow and enter the bloodstream.

Some types of blood cells only live for a few hours or a few days, so the bone marrow is constantly making new cells. The bone marrow normally makes millions of blood cells every day. This process is usually controlled - with cells dividing to produce new cells as they are needed.