Blood must not be mixed. Because if the blood groups do not match, the result can be deadly. There are 4 human blood types: O, A, B and AB. This is based on the presence or absence of antigens, molecules that trigger immune response: O has none, A has A antigen, B has B antigen and AB both A and B antigens.

That's why only type O blood can be safely donated to everybody. Put the wrong blood in a body that does not have those antigens and the result will be a massive immune activation, followed by a deadly hemolytic (blood break down) reaction, blood clotting, internal bleeding, dropping blood pressure and kidney failure. Blood mix-ups, even if rare (1:12,000), are still one of the most feared mistakes in medical transfusions. And to complicate the issue, just 7 % of the blood supply in US, for example, is made of type O blood.

But now a team at a Massachusetts biotech firm could have made a breakthrough in transfusion technology, ensuring a steady supply of type O blood to the nation's hospitals. This can be done using a device that turns all blood into type O.
The device work is based on the action of two enzymes recently found by Henrik Clausen of the University of Copenhagen in Denmark, that can cut off the antigen molecules from the surface of the red blood cells and can process eight units of type O each 90 minutes.

Clausen, a paid consultant to ZymeQuest, investigated through 2,500 bacteria and fungi species till he discovered in a gut bacterium, Bacteroides fragilis, an enzyme capable of breaking down the B antigen, and a second one, Elizabethkingia meningosepticum, which synthesize an enzyme that cuts off the A antigen. Human clinical trials on the A enzyme are on course in the U.S. and Europe.

This device would be also extremely helpful in case of trauma patients, when there's little time to find his/her blood type (and a wrong blood transfusion would kill, if the trauma did not).

The technology is still at the beginning and trials must show that the enzymes leave no unprocessed blood cells, as just a few wrong cells can trigger the immune reaction.