They could be anywhere in the world, but they are more likely to be in the UK.

We link with registers worldwide to find matches for people in the UK and provide stem cells for people in need abroad.

You’ll probably feel tired and have a little bruising after, but this generally passes within a week or so. You’ll receive lots of information and support for how to deal with them.

If you need to give cells from your bone marrow (found inside your large bones), doctors will give you a general anaesthetic and draw the cells from your pelvis using a sterile needle and syringe.

You will need to stay in hospital for two nights if you donate bone marrow.

Side effects can include flu-like symptoms and aching. But they’re usually mild and last just a couple of days. You’ll receive lots of information and support for how to deal with them.

On each of the four days before you donate, you will receive injections of granulocyte colony-stimulating factor (G-CSF). This is a naturally occurring hormone that increases the number of stem cells your body produces.

You’ll then come to one of our collection centres (in London, Oxford, Sheffield or Manchester), where a doctor will insert a tiny tube in your arm, draw out your blood, and pass it through a machine to collect the stem cells.

Donating only takes 4-5 hours for most people. At the end, a nurse will check how many stem cells you’ve donated. There is a chance that you’ll be asked to come back the next day to donate more.

You won’t need a general anaesthetic or to stay in hospital overnight.

You can donate your stem cells in two ways – through peripheral blood stem cell (PBSC) collection or bone marrow donation. Around 90% of people donate via PBSC collection. However, in some situations and for some conditions, a patient will need stem cells from bone marrow.

If you join our register, you must be willing to donate in either way.

The majority of people on our register never donate. Once you are on the register you have a 1 in 800 chance of being asked to donate in the next five years.

But your chance of being chosen to donate depends on your age and sex. A young man aged 16–30 has a 1 in 200 chance of being chosen to donate in the next five years.

We’ll only ask you to donate if you have a similar tissue type to someone in desperate need of a transplant.

If you’re a match and refuse, it could be devastating news for someone with a life-threatening illness. That’s why we ask you to be fully committed when you sign up.

No, you only need to join one stem cell bone marrow register. All potential donors in the UK are added to a single registry managed by Anthony Nolan. Joining more than one would cause duplication and waste valuable resources.

Anthony Nolan, The British Bone Marrow Register (operated by NHS Blood & Transplant), The Welsh Bone Marrow Donor Registry (operated by the Welsh Blood Service) and DKMS UK all recruit donors in the UK.

All these organisations add their donors to a single UK registry, the Anthony Nolan and NHS Stem Cell Registry, which Anthony Nolan manages.

They'll come back to Anthony Nolan and we find out your 'tissue type' in order to add your details to our register. Then every time someone needs a transplant, we'll compare their tissue type to yours - and to people on our register and registers across the world.

To find out more about how we work out your tissue type, head to The Science Behind Our Work.