Scientists have created red blood cells from human embryonic stem cells, in a step that they say could mean an infinite source of blood for transfusions.

According to the American Red Cross, 15 million units of blood are donated in the U.S. each year. Fourteen million units are transfused into Americans every year. The World Health Organization notes that people still die—especially in developing countries—because of an inadequate blood supply.

The team at Worcester, Mass.-based Advanced Cell Technology (ACT) says that if they can develop type-O negative blood—so-called "universal donor" blood because the body's immune system will not reject it—there could be an inexhaustible supply. They were able to grow type A, type B, and type O blood, but did not make type O negative. ACT's chief scientific officer Robert Lanza told Wired News that he doesn't think it will be a problem to make oodles of O negative.

Susan Shurin, deputy director of the National Heart, Lung and Blood Institute in Bethesda, Md., called the new work an "important first step."

"They've grown these in the test tubes and been able to get them to mature so that many aspects of these cells look like [red blood cells] that you make and I make," she told "But they haven't given them to people and see if they survive."

There are sugars on the surface of the cells, Shurin explains, that allow them to be recognized by the immune system. If the immune system sees them as risky, it could kill the cells. "There's still significant work to do," she says, "but it's a significant step in the right direction."

The American Red Cross agreed with Shurin's assessment, releasing a press statement, calling the work "pioneering," but also pointing out that the technique "has not progressed to the stage where the cultured cells are fully equivalent" to real red blood cells.

(Photo courtesy of Robert Lanza)