What are stem cells and bone marrow?

To understand stem cell transplants, it is helpful to know about stem cells and bone marrow.

About stem cell transplants

There are two different types of stem cell transplants:

To understand these treatments, it helps to know about bone marrow and stem cells.

Bone marrow is part of our immune system which protects us from infection and disease. It is found inside our bones, mainly in the hip bone and the breast bone. The bone marrow is where stem cells are made.

Stem cells are blood cells at the earliest stage of development. All our blood cells develop from stem cells in the bone marrow. Stem cells stay inside the bone marrow and when they are fully developed they go into the bloodstream.

Blood cells do not live long. The bone marrow normally makes millions of new blood cells every day to replace blood cells as they are needed.

There are three main types of blood cells:

  • Red blood cells contain haemoglobin (Hb), which carries oxygen from the lungs to all the cells in our body.
  • Platelets help the blood to clot and prevent bleeding and bruising.
  • White blood cells fight and prevent infection.

There are two main types of white blood cell. These are called neutrophils and lymphocytes. Neutrophils are the most common. You will hear your doctor or nurse talk about your neutrophil count during your treatment.

Bone marrow, stem cells and blood cells

Your blood count

Doctors measure the number of different blood cells by taking a blood test. They call this either a full blood count (FBC) or a blood count. Your blood count will be checked often during and after treatment. This is because the treatment will affect the levels of your blood cells.

The figures that show your blood cell levels look complicated when they are written down. But most people with cancer or leukaemia soon learn what they mean. For example, you may hear doctors or nurses saying things like "Your haemoglobin (Hb) is 135" or "Your neutrophils are 4".

The figures below show the levels of different blood cells usually found in a healthy person.

Type of blood cell


Red blood cells (Hb)

130–180g/l (men)
115–165g/l (women)


150–400 x 109/l

White blood cells (WBC)

4.0–11.0 x 109/l


2.0–7.5 x 109/l


1.5–4.5 x 109/l


These figures can be different from hospital to hospital. There may also be slight differences between people from different ethnic groups. Ask your doctor or nurse to explain more about your blood results if you are not sure.