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 obtaining 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.

The stages are described in more detail below.

Tests and examinations

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

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.

Harvesting stem cells

After you've had tests to check your general health, the stem cells that will be used for the transplant will 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 your blood using a special machine (see below)
• from bone marrow – where a procedure is carried out to remove a sample of bone marrow from the hip bone (see below)
• 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 from the NHS Cord Blood Bank)

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 isn't 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, medication that stimulates their production will be given for about 4 days beforehand. On the fifth 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 isn't painful and is done while you're awake. It takes around 3 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 won't 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 the needles were inserted (usually one on each side).

Conditioning treatment

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

• destroy existing bone marrow cells – this is 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 medication can be passed into your body without the need for lots of injections.

The conditioning process usually lasts up to a week. 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

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 won't be painful and you'll be awake throughout.

Recovery

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 vomiting, diarrhoea 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 not be able to leave hospital for longer.

Even after going home, you'll still be at risk of infections for potentially a year or 2 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 that stop your immune system from working so strongly, to reduce the risk of your body attacking the transplanted cells (immunosuppressants), or to reduce the risk of the transplanted cells attacking other cells in your body.

Stem cell or bone marrow transplants are complex treatments that carry a significant risk of serious complications.

It's important that you're aware of both the risks and possible benefits before treatment begins. You may wish to discuss them with your treatment team and your family.

Generally speaking, younger people who don't have any other serious conditions or those who receive transplants from a closely matched sibling are less likely to experience serious problems. People receiving transplants of their own stem cells (autologous transplants) are also less likely to experience serious side effects.

The main risks associated with a stem cell transplant are outlined below.

Graft versus host disease

In some cases, the transplanted cells recognise the recipient's cells as "foreign" and attack them. This is known as graft versus host disease (GvHD).

GvHD can occur within a few months of the transplant or develop several months or occasionally a year or 2 later. The condition is usually mild, but can sometimes be life-threatening.

Symptoms of GvHD can include:

• an itchy rash
• diarrhoea
• feeling and being sick
• a sensitive and dry mouth
• dry eyes
• dry, flaky skin
• shortness of breath
• joint pain
• yellowing of the skin and whites of the eyes (jaundice)

Tell your treatment team if you develop these symptoms. GvHD can be treated with medications that suppress your immune system and stop the transplanted stem cells attacking the rest of your body.

Reduced number of blood cells

In preparation for a stem cell transplant, you'll need to have chemotherapy to destroy the damaged or diseased blood cells. These will eventually be replaced by the transplanted stem cells, although this process can take several weeks or more.

Until your body starts being able to produce healthy blood cells again, you may be at risk of:

• anaemia – a lack of red blood cells that can make you feel tired and short of breath; this may be treated with regular blood transfusions
• excessive bleeding or bruising caused by a lack of clotting cells called platelets; you may need transfusions of platelets if this is a problem
• infections – a lack of white blood cells and any immunosuppressant medication you're taking will mean your body isn't able to fight off infections

You'll need to stay in a special germ-free hospital room during the first few weeks after the transplant. After going home, you'll need to be careful about coming into contact with people with infections and take steps to prevent food poisoning.

You may also be given antibiotics to prevent or treat any bacterial infections.

Chemotherapy side effects

Common side effects of chemotherapy include:

• feeling and being sick
• diarrhoea
• loss of appetite
• mouth ulcers 
• tiredness
• rashes
• hair loss 

These side effects are usually temporary and only last a few weeks. Hair usually grows back within a few months.

However, high-dose chemotherapy can also have some long-lasting effects, including permanent infertility, which affects most people who have the treatment.

Your treatment team will tell you about this before treatment starts if it's a risk and they can discuss possible ways of having children in the future. In some people this may include procedures to collect and freeze eggs or sperm, although this isn't always possible. Read more about cancer and fertility.