Don: Wow, that is exciting. But what makes type O blood so special?

Y: Blood types result from different carbohydrates, called antigens(抗原), on the surface of human blood cells. Your blood "type" denotes the kinds of antigens you have on the surface of your cells. People with type A blood have A antigens; those with type B have B antigens. If you have both kinds of antigens, you're type AB; if you have neither one, you're type O.

Your immune system recognizes the antigens on your blood as "self", but attack cells with "foreign" antigens. So if you ever need a blood transfusion, it's important that you only receive blood compatible with your own type.

D: I see. So if a type A person receives type B or AB blood, her immune system will see the B-antigens as foreign, and attack the donor blood cells?

Y: Exactly. But type O is called the "universal donor" since it doesn't have any antigens that would set off the body's defense system.

A team of Danish scientists has identified enzymes that remove the A and B antigens from blood cells. The enzymes, which were extracted from a bacteria and a fungus, essentially change any donated blood into type O.

Clinical trials are underway to test whether the treated blood is safe and effective. If so, the technology could help save lives, since type O blood is such a precious commodity.