British scientists have successfully regenerated a living organ for the first time.

The research team built a thymus, a specialized organ in the immune system that produces T cells, by instructing stem cells to rebuild the tissue. The study, published in the journal Development, was conducted on mouse tissue, in which researchers found a way to reactivate a natural mechanism that usually shuts down with age.

The protein that was targeted, called FOXN1, shuts down as the thymus ages. The team restored it to levels found in younger mice by increasing the activity of that particular protein. While the study was conducted on mice, scientists say it may have broader implications for humans.

"Our results suggest that targeting the same pathway in humans may improve thymus function and therefore boost immunity in elderly patients, or those with a suppressed immune system,” Clare Blackburn from the MRC Centre for Regenerative Medicine at the University of Edinburgh said in a statement. She added that more research needs to be done before tests can be conducted on humans.

The team increased the FOXN1 levels in mice using chemical signals. This allowed "stem cell-like" cells in the thymus to rebuild the organ in older mice.

"One of the key goals in regenerative medicine is harnessing the body's own repair mechanisms and manipulating these in a controlled way to treat disease,” Dr. Rob Buckle, the head of regenerative medicine at the Medical Research Council, said. “This interesting study suggests that organ regeneration in a mammal can be directed by manipulation of a single protein, which is likely to have broad implications for other areas of regenerative biology."

In humans, the thymus sits near the heart and shrinks to a tenth of its original size by adolescence. While the exact cause of the shrinkage is unknown, some scientists say the body might redirect some of the energy used to run to the organ to reproductive organs as the body ages.

"This has a lot of impacts later in life, when the functionality of the immune system decreases with age and you become more vulnerable to infection and less responsive to vaccines," Dr. Nick Bredenkamp, one of the researchers, told the BBC.

However, keeping the thymus the same size throughout life may have harmful consequences to humans. For instance, if the thymus’ growth isn’t halted, it could overreact and attack the body.