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What is a stem cell?


If you’ve been told that you need a stem cell transplant, one of your first thoughts might be: but what is a stem cell?

On this page we’ll explain what stem cells are, why they are important, how they work, and why you might need a stem cell transplant. 

What are stem cells?

Stem cells are immature cells in your body. This means they can divide and develop into all the different types of specialised cells your body needs.  

Specialised cells make everything from your organs and muscles, to your nerves and blood. All specialised cells start as stem cells.

Here, we’ll talk about the stem cells which create your blood cells. 

Blood stem cells are cells which are found in the bone marrow. Your bone marrow is spongy tissue found in the middle of some bones, like your hip, thigh and breastbone. 

Blood stem cells can become three main types of blood cell: 

  • Red blood cells, which carry oxygen around the body
  • White blood cells, which help to fight infections
  • Platelets, which help to form blood clots and stop bleeding
Illustration of a blood stem cell

Once a new blood cell is made, it travels into your bloodstream to do its important work. Some stem cells can travel into your bloodstream too. This is how stem cells are collected from someone during a peripheral blood stem cell (PBSC) donation – a process a bit like giving blood.  

Why are stem cells important?

You need healthy blood cells to survive. But blood cells only live for a short amount of time – some for just a few hours and some for about 120 days (four months). This means that they need to be replaced quickly and often by the blood stem cells in your bone marrow.  

Millions of new blood cells are made in your body every day!  

How do stem cells work?

Blood stem cells are like templates with instructions for creating new blood cells. They create the millions of blood cells needed in your body every day by copying themselves over and over again.  

If one copies (also known as divides) itself incorrectly, this can lead to millions of ‘wrong’ templates and instructions. If this happens, your new blood cells may not work properly, and you could develop a blood cancer or a blood disorder.  

How does stem cell division go wrong?

You can find out more about how the process of cell division can go wrong on the Cancer Research UK website. 

The webpage below is about cancer in general, and isn’t specific to blood cancer and blood disorders, but may help your overall understanding.

When is a stem cell transplant used?

A stem cell transplant is often the last treatment option if you have a condition which affects your blood cells or bone marrow. This might be a blood cancer like leukaemia or lymphoma, or a blood disorder like aplastic anaemia. 

If you’re diagnosed with a blood cancer or a blood disorder, you may have treatment such as chemotherapy to destroy the broken blood cells and blood stem cells. If this treatment doesn’t work, you may need to have a stem cell transplant. 

A stem cell transplant will replace your damaged stem cells with healthy ones. You can find out more about what a stem cell transplant involves on our What is a stem cell transplant? webpage. 

What is stem cell therapy?

Stem cell therapy is not the same as stem cell transplant.  

Stem cell therapy uses stem cells to repair damaged cells in the body. It’s a developing treatment undergoing lots of research – there’s still lots we don’t know – but it could be an option in the future for people with conditions such as multiple sclerosis (MS) and Parkinson’s disease.  

Stem cell therapy doesn’t necessarily provide a cure, like a stem cell transplant, but could ease symptoms, slow progression of a disease, and give a better quality of life.  

Anthony Nolan supports patients who are having a stem cell transplant, not stem cell therapy. 

Information published: 09/02/2022
Next review due: 09/02/2025

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