Scientists have developed a way of converting one blood group into another.
The technique potentially enables blood from groups A, B and AB to be converted into group O negative, which can be safely transplanted into any patient.
The method, which makes use of newly discovered enzymes, may help relieve shortages of blood for transfusions.
The work, led by the University of Copenhagen, is reported in the journal Nature Biotechnology.
Using incompatible blood during a transfusion can put a patient's life in danger.
The blood cells of people with group A and B blood contain one of two different sugar molecules, which act as "antigens", triggering an immune system response.
People with AB blood have both types of molecule, while those with group O blood have neither.
People produce antibodies against the antigens they lack.
This means groups A, B and AB can only be given to patients with compatible blood, while O - as long as it is rhesus negative - can be given to anyone.
The new technique works by using bacterial enzymes to cut sugar molecules from the surface of red blood cells.
After a search of 2,500 fungi and bacteria the researchers discovered two bacteria - Elizabethkingia meningosepticum and Bacterioides fragilis - which contained potentially useful enzymes.
They found that enzymes from both bacteria were able to remove both A and B antigens from red blood cells.
Trials needed
However, they say that patient trials will be needed before the conversion method can be used in hospitals.
Writing in the same journal, blood experts Geoff Daniels, of the Bristol Institute for Transfusion Sciences, and Stephen Withers, of the University of British Columbia, Canada, welcome the research.
They said the use of enzymes to convert blood group has long been proposed, but has proved to be impractical due to the inefficiency and incompatibility of available enzymes.
However, they say the enzymes discovered in the latest study may finally overcome these problems.
They write: "Their method may enable manufacture of universal red cells, which would substantially reduce pressure on the blood supply."
The new process cannot do anything about another antigen that can trigger an immune response. Blood which carries this antigen is known as rhesus positive.
This means that only rhesus negative blood can be used to create the new type of group O supplies.
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