Danish researchers have discovered two enzymes that could convert type A, type B, and type AB blood all to type O, the universal donor.
Blood type is determined by the presence of antigens on the surface of red blood cells. People with antigen A are type A, people with antigen B are type B, and people with both are type AB. If a person with type O blood (which contains neither antigen) received type A blood, their immune system would interpret the blood cell as foreign and attack it. However if a person with type A blood received type O blood, there are no antigens to recognize and the blood would be accepted. This is the reason that anyone can receive type O blood (and conversely type AB people can receive any blood - they don't recognize any antigens as being foreign). There is also another important antibody, the presence or absence of which is denoted by a "positive" or "negative" designation.
The two enzymes reported essentially destroy the two antigens, converting the blood to type O. Interestingly, they were discovered by screening bacteria and fungi for enzymes capable of this activity. Many drugs we are accustomed to (for instance Tylenol, Viagra and Lipitor) work by inhibiting a particular protein that when left to function produces some undesirable effect. In this case the drugs are small molecules, and therefore can be rationally designed for that purpose. Enzymes are far more complicated however, and therefore are resistant to rational design. Instead, this screening technique is commonly used.
Small molecule drugs can also be found by this approach; in fact, this is where most antibiotics came from. My current boss, Scott Strobel, just got back from the jungles of Peru where he took undergrads to look for natural products. Maybe the next Tylenol is sitting in the dirty cardboard box in our fridge.