Ava Stark, from Lochgelly, lost out on a life-saving transplant for the second time in four months after a donor pulled out for medical reasons.
The little girl has a rare blood disorder and will die without a stem cell transplant. The donor has to be a 10 out of 10 identical genetic markers match.
A fresh appeal, made this week after Ava’s transplant fell through, has generated a massive response as ten of thousands signed up with bone marrow charities.
People have registered to donate their blood cells by filling out a form and providing a saliva sample.
If their stem cells are needed they have a procedure which is similar to giving blood.
A previous search of the 25 million donors on the global register, following Ava’s diagnosis in April with inherited bone marrow failure, revealed just one match.
The donor agreed to help Ava with the operation, scheduled for July, but was later unable to go ahead.
The family were relieved when a second match was found and were due to take Ava to hospital in Glasgow on Tuesday to get ready for her transplant.
But they were “devastated” when doctors said the new donor had to pull out just hours before due to medical reasons.
Any relatively healthy adult aged between 16 and 30 can sign up for the painless test through the Anthony Nolan Trust while older donors can register through Delete Blood Cancer (DKMS), which takes people aged 17-55.
Ann O’Leary, head of regional register development at Anthony Nolan, told the BBC that they had received more than 16,000 new registrations in just 48 hours.
She said Ava had been “incredibly courageous and positive”.
Ms O’Leary said: “A stem cell transplant is Ava’s best chance of life, but without a match that can’t happen.
“What many people don’t realise is how easy it is to join - it simply involves filling in a form and providing a saliva sample.
“If you’re one of the privileged few who goes on to donate, 90% of the time this will take place via a straightforward outpatient procedure.
“The public’s response to Ava’s story has been overwhelming and shows just how incredible and selfless people can be when they rally together to save the life a stranger.”
Sarah Gray, DKMS donor recruitment manager, said they had received more than 33,000 new registrations in two days.
She said: “The response to Ava’s situation has been incredible.
“Tens of thousands of people have been so moved by what has happened to Ava, and all she has been through, that they have now registered to see if they could be a matching blood stem cell donor for her or another person seeking a donor to save their life.
“As you can stay registered until you are 61, and potentially match with and help save the life of a patient until then, the impact of this response will last decades.
“While this has been a great response, we need many more people to register. So please register, for Ava, and everyone who needs a blood stem cell donation for their survival.”