Blood types are a lot more complicated than A, B, AB and O. As scientists discover dozens more genetic variations in human blood markers, hospitals and blood banks across the country are redefining how they screen the national blood supply, the Wall Street Journal reports.

Researchers said there are actually at least 33 unique blood types, as well as 300 different antigens. (Antigens are genetic protein markers that may be present on human blood cells.) That’s important, because in the blood donation business, every mismatched antigen can raise a recipient’s likelihood of a life-threatening immune response.

Doctors noted that most blood transfusions are safe using matching for the four basic types. But for people who need multiple transfusions, such as patients with cancer, intestinal bleeding or sickle cell anemia, problems can arise because they must have a more precise genetic match.

To address this problem, the U.S. Food and Drug Administration recently approved a new blood test, called PreciseType. The test screens for up to 35 antigens in a person’s blood that could trigger immune reactions. What’s more, researchers have recently started to push hospitals and blood banks to match recipients and donors with the same ethnic background to help minimize these potential risks.

Currently, 15 health systems across the country as well as 25 donor centers, including the American Red Cross, have adopted the new blood test. In addition, efforts are also underway at the U.S. Centers for Disease Control and Prevention to help increase minority blood donation.

Besides minority blood donors, U.S. hospitals also desperately need African Americans to donate bone marrow. Click here for more information.