A stem cell or bone marrow transplant is a long and complicated process that involves 5 main stages.

These stages are:

1. Tests and examinations – to assess your general level of health.
2. Harvesting – the process of collecting the stem cells to be used in the transplant, either from you or a donor.
3. Conditioning – treatment to prepare your body for the transplant.
4. Transplanting the stem cells.
5. Recovery – you'll need to stay in hospital for at least a few weeks until the transplant starts to take effect.

Before a stem cell transplant can be carried out, you'll need a series of tests and examinations to ensure you're healthy enough for the procedure to be carried out.

Transplants tend to be more successful in people who are in good general health, despite their underlying condition.

The tests you might have include:

• an electrocardiogram (ECG) – a simple test used to check your heart's rhythm and electrical activity
• an echocardiogram – a scan used to look at the heart and nearby blood vessels
• an X-ray and/or computerised tomography (CT) scan to check the condition of organs such as the lungs and liver
• blood tests to check the level of blood cells and assess how well the liver and kidneys are working
• a coronavirus (COVID-19) test – both you and your donor may be tested for COVID-19 and if either test is positive the transplant may be delayed

If you have cancer, you may also need to have a biopsy. This is where a small sample of cancerous cells is removed and analysed. It can show whether your cancer is under control (in remission) and whether there's a high risk of it returning after your transplant.

After you've had tests to check your general health, the stem cells that will be used for the transplant need to be removed and stored.

There are 3 main ways stem cells can be harvested. These are:

• from blood – where the stem cells are removed from the blood using a special machine
• from bone marrow – where a procedure is carried out to remove a sample of bone marrow from the hip bone
• from cord blood – where donated blood from the placenta and umbilical cord of a newborn baby is used as the source of stem cells (find out more about cord blood on the NHS Blood and Transplant website)

It may be possible to remove stem cells from your own blood or bone marrow and transplant them later after any damaged or cancerous cells have been removed.

If this is not possible, stem cells from a donor's blood or bone marrow will usually be used.

Removing stem cells from blood

The most common way to harvest stem cells involves temporarily removing blood from the body, separating out the stem cells, and then returning the blood to the body.

To boost the number of stem cells in the blood, medicine that stimulates their production will be given for about 4 days beforehand. On the 5th day, a blood test will be carried out to check there are enough circulating stem cells.

If there are enough cells, veins in each arm will be connected by tubes to a cell separator machine. Blood is removed from one arm and passed through a filter, before being returned to the body through the other arm.

This procedure is not painful and is done while you're awake. It takes around 3 to 4 hours and may need to be repeated the next day if not enough cells are removed the first time.

Removing a bone marrow sample

An alternative method of collecting stem cells is to remove around a litre of bone marrow from your hip bone using a needle and syringe.

The needle may need to be inserted into several parts of your hip to ensure enough bone marrow is obtained. This is done under a general anaesthetic, so you'll be asleep and will not feel any pain while it's carried out.

However, the area where the needle is inserted may be painful afterwards and you'll have marks on your skin where it was inserted (usually one on each side).

Treatment with high doses of chemotherapy and sometimes radiotherapy will be needed before the stem cells can be transplanted.

This is to:

• destroy existing bone marrow cells – to make room for the transplanted tissue
• destroy any existing cancer cells
• stop your immune system working – this reduces the risk of the transplant being rejected

As part of the conditioning treatment, you'll be given a range of medicines, so a tube called a central line will usually be inserted into a large vein near your heart. This means medicine can be passed into your body without the need for lots of injections.

The conditioning process usually lasts about a week or 2. You'll probably need to stay in hospital throughout the treatment.

Conditioning can cause a number of unpleasant side effects, such as sickness, hair loss and tiredness. These are usually temporary. Your treatment team will discuss the risks of treatment with you beforehand.

Read more about the risks of stem cell transplants.

The transplant will usually be carried out a day or 2 after conditioning has finished.

The stem cells will be passed slowly into your body through the central line. This process often takes around a couple of hours.

The transplant will not be painful and you'll be awake throughout.

Once the transplant is finished, you'll need to stay in hospital for a few weeks while you wait for the stem cells to settle in your bone marrow and start producing new blood cells.

During this period you may:

• feel weak, and you may experience diarrhoea and vomiting, and/or a loss of appetite
• be given fluids by mouth or through a tube running from your nose to your stomach (a nasogastric tube) to prevent malnutrition
• have regular blood transfusions, as you'll have a low number of red blood cells
• have regular platelet transfusions, as you'll have a low number of platelets
• stay in a special germ-free room, and visitors may need to wear protective clothing to prevent infections, as you'll have a low number of infection-fighting white blood cells

Many people are well enough to leave hospital between 1 and 3 months after the transplant. However, if you develop complications such as an infection, you may have to stay in hospital for longer.

Even after going home, you'll still be at risk of infections for potentially a year or longer because it can take a while for your immune system to return to full strength.

If donated stem cells were transplanted, you'll also usually need to take medicines called immunosuppressants that stop your immune system from working so strongly. This is to reduce the risk of your body attacking the transplanted cells or to reduce the risk of the transplanted cells attacking other cells in your body.