What would I say to the stranger who saved my life?

How could I ever thank this total stranger who helped me in a way that no one else could?

The other day, I received a card from the man I've been dreaming about for months. My perfect match. My saviour.

This might sound like romantic hyperbole, but it's true. I'm talking about my stem-cell donor, who is a 10/10 match for me, based on our HLA markers (human leukocyte antigen - the unique genetic "barcode" on all of our immune systems' cells). It's no exaggeration to say that he has saved my life.

When I was diagnosed with leukaemia last year aged 22, doctors told me that I urgently needed a stem cell transplant. Previously known as a bone marrow transplant, a stem cell transplant essentially involves destroying the patient's faulty immune system with high-dose chemotherapy and re-installing a brand new version, using someone else's stem cells.

When tests showed that my sister was not a match for my tissue type (only 37 per cent of UK patients find a match in their families), the hospital began combing the bone marrow register for an unrelated donor.

The doctors warned me that a match might never be found, and with my mixed ethnic background (my father is Jewish-Hungarian and my mother is Croatian), the outlook was less encouraging.

So as I underwent two cycles of aggressive chemotherapy, I waited in my isolated hospital room, eaten up with fear, unable to sleep at night. I knew that my life was dependent on a stranger having signed up to the register.

I remember my 23rd birthday - the giggling outside my door, followed by a troupe of nurses bursting in singing Happy Birthday, wheeling a cake on a trolley. I managed to fake a smile, but I couldn't silence the voice in my head, telling me it was probably the last time I would be blowing out candles on a cake. I didn't think I would make it to 24.

But luck was on my side. A donor was found through the Anthony Nolan bone marrow charity; a young man a year older than me, who generously donated six million of his cells. This is all that I have been told about him.

There is a strict anonymity policy between donors and recipients for two years after the transplant, at which point we're allowed to exchange identities if we wish. I sent him a thank-you card a while ago, and the other day I finally heard back from him. I can't say much else - I'm not allowed to reveal anything that might identify him. Maybe one day we will meet.

But what do you say to someone who's saved your life? How could I ever thank this total stranger who helped me in a way that no one else could? Because beyond all the medical care I've received, the advanced treatment and the teams of highly-trained doctors and nurses, my donor is the single piece in the jigsaw puzzle that nothing else could work without. A real-life, breathing human being who underwent growth-hormone injections for several days (to boost his cell count) and sat for hours in a London clinic hooked up to an apheresis machine which "harvested" his stem cells. All for someone he's never met.

Recently, I heard about someone facing a similar dilemma - 65 year-old Peter Threader from Stockport, and I was lucky enough to be there when he met his donor for the first time.

After a routine blood test three years ago, Peter was diagnosed with hypoplastic myelodysplasia - a disorder caused by the bone marrow failing to produce enough healthy blood cells, resulting in severe anaemia, frequent infections and other complications.

"I was shocked," he says. "I felt completely well at the time. I was running up and down the stairs." But doctors told him that his condition would deteriorate dramatically unless he had a transplant. As in my case, his sister was not a match, so an unrelated donor was found - Dave Maguire, now 28, from North Wales.

For the most part, Peter is stoical about his travails, but his voice breaks when he talks about his donor's act of altruism, and he is close to tears as he recalls the first time he received a card from Dave. At the time, Peter was in hospital recovering from pneumonia (infections are common in patients post-transplant, due to the immunosuppressant medication they are given to prevent their bodies rejecting the donors' cells.) He had just been discharged from intensive care and was lying in bed on the transplant unit, unable to walk. "I remember a nurse came in with a card and said 'this'll cheer you up'." The card was a response to a letter Peter had sent: "I'd written 'thank-you for your stem cells'. 'No problem' was the reply. 'There's always more if you need them'."

For two years, Peter and Dave continued their correspondence, always beginning their messages with "Dear donor" or "Dear recipient". "It was strange, not knowing who I was writing to," says Peter. "It'll be good to finally put a face to the name." Dave is excited but admits that he's a bit nervous. Both men are conscious of the fact that they're total strangers to one another (albeit strangers who now share the same bone marrow and DNA - Peter's blood group has even switched to Dave's.)

As it turns out, when they meet in Dave's home town of Rhyl, there are no awkward silences. Dave has brought along three generations of family members and his best friend, Silvana (she travelled to London with him when he donated his cells). Peter is accompanied by his wife, Mary, who instantly wraps Dave in a hug. "Thank-you," she says simply, "for saving my husband's life."

Over a traditional Sunday dinner, the Threaders share stories with the Maguire clan and laugh over the unexpected side-effects that Dave's stem cells have had on Peter. As a transplant patient takes on their donor's immune system, they typically inherit the various allergies and conditions that come with it - happily, Peter has been cured of his hayfever and haemochromatosis (a disorder which results in an overload of iron in the blood), but he says that he's now more forgetful. "Sorry," says Dave with a smile. "That's probably my fault."

He is an apologetic sort of lifesaver - he certainly doesn't want any thanks for what he's done. "My bone marrow couldn't have gone to a nicer person," he tells me when the day is over. "I feel like saying 'don't worry if you need anything else: kidneys, liver, you can have them all!"'

Dave has been on the donor register since the age of 16 and he was thrilled when Anthony Nolan contacted him nearly a decade later, inviting him to donate his cells. But it was seeing photos of Peter's young grandchildren when they met (he has four, aged between two and 14), that really convinced him what the transplant had achieved. "There are no words," he says. "I can only hope that if my granddad was in the same position, there would be someone out there who could do the same for him."

Anyone can suddenly be faced with a life-threatening diagnosis. As I'm all too aware, cancer does not discriminate. No matter how fit, young or healthy you are, you can end up in a hospital ward, caught in a horror-film scenario with your life hanging in the balance. For around 1,800 people in the UK each year, a stem cell transplant is their only chance of survival. But according to the NHS Stem Cell Strategic Forum, over 400 patients a year are unable to have the treatment they need, simply because a match is never found. Ethnic minorities and patients from mixed-race backgrounds are particularly disadvantaged, as they are seriously under-represented on bone marrow registers. Potential recruits are also deterred by outdated myths; many people are unaware that, 90 per cent of the time, donating stem cells is very similar to giving blood - there's no painful bone-drilling operation required any more. ("It was easy," Dave tells me - he was even well enough to go out clubbing after donating).

Today, I heard that a friend has died; a match was never found for her. It didn't come as a surprise, but it shocked me all the same - a death that could perhaps have been prevented had there been more people like Dave, more people like my donor, whoever he is.

I don't know what I'll say if I ever meet him. I just wish there were more heroes out there like him.