Most people know their blood type is either A, B, AB or O, and may even know if it's positive or negative, but new research from the U.S. has uncovered two more categories.

Langereis blood type and Junior blood type are rare, but they do exist in several ethnic populations.

"More than 50,000 Japanese are thought to be Junior negative and may encounter blood transfusion problems or mother-fetus incompatibility," University of Vermont biologist Bryan Ballif wrote in his study, which appears in this month's issue of Nature Genetics.

Ballif and his fellow researchers discovered two proteins on red blood cells responsible for these lesser-known blood types.

Ballif identified the two molecules as specialized transport proteins named ABCB6 and ABCG2.

This science may be especially important to organ transplant patients, he said.

"As we get better and better at transplants, we do everything we can to make a good match," Ballif said in a release about his study, noting sometimes, even though everything looks good, the body rejects the organ.

"We don't always know why there is rejection," Ballif said. "It may have to do with these proteins."