The feat came as part of an ongoing study being carried out by scientists at the universities of Cambridge and Bristol as well as researchers from the U.K.’s National Health Service (NHS), which is comparing the life span of lab-grown blood with standard blood that has come from the same donor.

Is it safe?

The manufactured blood cells—grown from stem cells that were collected from donors—have so far been transfused into two people, the research team behind the study announced Monday.

It marks the first time lab-grown blood cells have been put into the body of someone who is not the donor of the cells from which the blood was grown, scientists said.

Around one to two teaspoons, or 5 to 10 milliliters, of lab-grown blood was transfused into the participants. In a regular blood transfusion, patients are given much more blood than this.

The recipients of the blood were being closely monitored, researchers noted, and no “untoward” side effects had been reported. “They are well and healthy,” experts said.

At least 10 participants will receive two “mini transfusions” of blood—one consisting of lab-grown blood, one of standard donated blood—as the trial progresses, which will be given a minimum of four months apart.

Scientists will analyze whether the “young” red blood cells made in the lab last longer than cells made in the body.

Better than “real” blood?

Blood transfusions can be lifesaving procedures and are used in various circumstances, from treating people who have suffered severe bleeding after childbirth or a serious accident to treating those with sickle cell disease and certain types of cancer.

Experts working on the study said that because the lab-grown blood cells were all new, while blood collected directly from donors was made up of cells of varying ages, they expected the lab-grown blood to have longer life spans.

In a video outlining the trial and its aims, professor Ashley Toye of the University of Bristol and NHS Blood and Transplant’s research unit said the team was hopeful that because the lab-made cells were “so freshly made and ready to go,” they would perform better.

Potential to “revolutionize” treatments

Researchers acknowledged on Monday that further trials were needed before lab-grown blood could be rolled out for medical use, but they said their study marked a “significant” step toward using it to improve treatments for patients with “complex transfusion needs.”

“If proved safe and effective, manufactured blood cells could in time revolutionize treatments for people with blood disorders such as sickle cell and rare blood types,” they said in a press release.

“It can be difficult to find enough well-matched donated blood for some people with these disorders. Additionally, if manufactured cells last longer in the body, patients who regularly need blood may not need transfusions as often. That would reduce iron overload from frequent blood transfusions, which can lead to serious complications.”

Farrukh Shah, medical director of transfusion for NHS Blood and Transplant, explained in a statement on Monday that patients who need regular or intermittent blood transfusions can develop antibodies against minor blood groups, making it more difficult to find donor blood that can be transfused into those individuals without the risk of a potentially life-threatening reaction.

“This world-leading research lays the groundwork for the manufacture of red blood cells that can safely be used to transfuse people with disorders like sickle cell,” she said. “The need for normal blood donations to provide the vast majority of blood will remain. But the potential for this work to benefit hard-to-transfuse patients is very significant.”

John James, chief executive of the U.K.’s Sickle Cell Society, said in a statement that while the research offered “real hope,” it was important to remember that Britain’s NHS still relied on 250 blood donations per day to treat people with sickle cell disorder.

“The need for normal blood donations to provide the vast majority of blood transfusions will remain,” he said.