Scientists believe they have found a reliable way to transform donor blood into the universal type needed for safe, emergency blood transfusions.
The discovery is enzymes from gut bacteria that can efficiently turn type-A human blood into type-O.
Type-O blood is special because it can be donated to anyone without the risk of a bad mismatch reaction.
The researchers, from the University of British Columbia, say clinical trials of the treatment could begin soon.
The gut bug enzymes remove markers from the surface of the donor red blood cells present in type A but not in type O.
Stripping them away means the recipient's immune system will think the donor type-A blood is type-O and will not attack it for being "foreign".
Giving someone blood from the wrong group can be life-threatening.
While type-O patients can receive only type-O red cell transfusions, type-O donations are compatible with all other ABO types.
Blood groups
- The four main blood groups in the ABO system are: O, A, B and AB
- Which group you belong to is determined by the genes you inherit from your parents
In an emergency situation, there may not be time to do a full check to get a good match, which is why having enough type-O blood to use is so important for health services.
About 7-8% of the UK population have type-O Rhesus-negative blood but demand for this special group accounts for about 13% of all hospital requests, according to NHS Blood and Transplant.
Beth Johnson, 18, from Irlam, Lancashire, is a blood donor and her donation was used to help people injured on the night of the Manchester Arena attack, 22 May 2017.
"I am O-negative and know how rare and valuable this blood type is for the NHS," she said.
"I'm so glad my blood was potentially used to save the life of someone involved in something so horrific."
The idea of changing blood type with enzymes is not new – other scientists as well as the British Columbia team have been exploring it for some time.
But researcher Stephen Withers, who is presenting his findings at the American Chemical Society's national meeting in Boston, said the gut enzymes represented the most promising treatment so far.
He said: "I am optimistic that we have a very interesting candidate to adjust donated blood to a common type.
"Of course, it will have to go through lots of clinical trails to make sure that it doesn't have any adverse consequences but it is looking very promising.
"It works in whole blood, so you could see this being put into the bag at the time of collection and just sitting there doing its job while this stuff is being stored."
In laboratory tests, the enzymes were able to completely convert blood type A to O.
With the help of the Canadian Blood Service, the researchers now want to test more blood samples before trialling the treatment in the clinic.
"Obviously, the next stages are all about safety, making sure this doesn't cause any inadvertent effects," Mr Withers said.
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