What is a stem cell?

If you’ve found out that you need a stem cell transplant, one of your first thoughts might be ‘what is a stem cell?’ On this page we’ll explain what stem cells are, why they’re important, and why you might need a stem cell transplant.

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What are stem cells?

Stem cells have an important role in your body. They are a type of immature cell which can divide and develop into all the different types of specialised cells your body needs.

All specialised cells start as stem cells. You need specialised cells to make everything in your body – from your organs and muscles, to your nerves and blood. Stem cells act like templates with instructions for creating new cells.

Here, we’ll talk about the stem cells which create your blood cells. You can find blood stem cells in your bone marrow. Your bone marrow is spongy tissue found in the middle of some bones, like your hip, thigh and breastbone.

Why are stem cells important?

Stem cells are important because we need to replace our blood cells quickly and often, which we cannot do without stem cells.

We all need lots of healthy blood cells to survive. But blood cells only exist for a short amount of time – some for just a few hours and others for about 120 days, which is four months. Your body needs to make millions of new blood cells every day!

What do different types of blood cells do?

You have various types of blood cells in your body. They all have different names and roles in keeping the body healthy:

  • Red blood cells are the most common type of blood cell. Red blood cells carry oxygen all around your body. You might also hear them called ‘erythrocytes’. If you have too few red blood cells or they don’t work properly, it can cause anaemia.
  • Platelets help the blood to clot after an injury or trauma.
  • White blood cells make up your immune system and fight infections. We can sort white blood cells into smaller groups based on the jobs they do:
    • Neutrophils – one of the first cell types to respond to infections. Neutrophils are the most common type of white blood cell. You might experience inflammation and swelling when they go to the site of an infection.
    • Lymphocytes – there are two main types of these white blood cells. T cells identify cells that shouldn’t be in your body and gather other immune cells to help. B cells produce antibodies.
    • Plasma cells – mature B cells that produce different types of antibodies to help fight infection. When problems occur with their production and development, it can lead to a type of blood cancer called myeloma.

Once your body makes a new blood cell, it travels into your bloodstream to do its important work. Some stem cells can travel into your bloodstream too. This is how doctors collect stem cells from a donor in a peripheral blood stem cell (PBSC) donation – a process a bit like giving blood.

Illustration of a blood stem cell

How do stem cells work?

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

However, if a stem cell copies itself incorrectly, this can lead to millions of ‘wrong’ templates and instructions. If this happens, your new blood cells may not work properly. This is why some people develop a blood cancer or a blood disorder.

How does stem cell division go wrong?

Cancer Research UK offers more information about how the process of cell division can go wrong. Their information talks about cancer in general – it isn’t specific to blood cancer and blood disorders, but may help you to better understand.

When is a stem cell transplant used?

If you have a condition which affects your blood cells or bone marrow, a stem cell transplant is often the last treatment option. This includes conditions such as:

If you get a blood cancer or blood disorder diagnosis, you may have treatment like chemotherapy. This aims to destroy the damaged blood cells and blood stem cells. If chemotherapy doesn’t work, you may need to have a stem cell transplant to replace your damaged stem cells with healthy ones.

To find out what a transplant involves, see our page What is a stem cell transplant?

Information updated: 23/05/2024

Next review due: 23/05/2027