A mum and dad's crushing letter: "Every breath Imogen takes is a breath for our dead son."

Imogen Bolton was just a few months old when her parents, Hayley and Jason, received devastating news. Their tiny daughter had the rare condition alveolar capillary dysplasia, meaning her lungs weren’t fully formed. She was only expected to live for a few weeks.

“When we first asked about a transplant they said she was too young and too small,” her mother told The Sun. “Then they told us the biggest issue would be finding a donor.”

Without much hope, Imogen was put on the transplant list. Miraculously, just one week later, a suitable donor was found. At the age of five months, Imogen became the youngest person in the UK to undergo a double lung transplant.

With her new lungs, Imogen thrived. After she was released from hospital, her parents went public with her story in October. Her mother said they were “hugely grateful” to the donor family.

“We don’t know who they are, but we are able to write to them,” she said. “I’ve started the letter several times, but I feel grief. They lost their baby and it could have been me.”

Imogen with her parents Jason and Hayley. Image via ITV News/screenshot. 

Now, the donor's parents have come forward with their story. They want to remain anonymous, but they have revealed that their son, Theo, died when he was just 41 days old after suddenly falling ill.

"We search for our son’s presence everywhere and every day," they wrote in an open letter.

“Now we have known one of the brightest places in Imogen. For that we thank Imogen and her family for sharing this special story. A once-in-a-lifetime chance meeting of two little people bravely and beautifully fighting for life."

For them, it wasn't a difficult decision to support Theo to make his donation.

“We believe he would have wanted to help others if he had been able to grow up and make the decision himself.

“We are proud of what Theo could do not just for Imogen, but also for a young adult who now runs their body with his two tiny kidneys."

A younger Imogen in hospital. Image via ITV News/screenshot.

The parents say that every breath Imogen takes is a breath for their son.

“We know that every breath Imogen takes is a breath for our son," they wrote.

“Every birthday Imogen celebrates is also a celebration of Theo’s birth. We imagine how perhaps someday Imogen and Theo can blow out his birthday candles together."

Hayley and Jason Bolton know that their daughter wouldn't be alive if it wasn't for Theo's "incredible gift".

“Theo lives on through Imogen," they say, "and when she hits her different milestones, our family will be celebrating not one life, but two.”