The team of researchers from Edinburgh University managed to rejuvenate a thymus in a mouse after it was "shut down" by old age.
The group from the university's Medical Research Council (MRC) Centre for Regenerative Medicine used chemical science to manipulate the organ's genetic structure by switching stem cells within the thymus back on.
They were then able to regrow the organ back to twice its depleted size and restart its production of virus-fighting white blood cells.
The research raises the possibility that one day similar techniques could be used to regenerate hearts, livers and lungs after they have been ravaged by age or disease.
Dr Rob Buckle, head of regenerative medicine at the MRC, said: "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.
"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."
The thymus is found in front of the heart and forms a vital cog in an animal's immune system.
It is the first part of the body to significantly degenerate as an animal grows older.
By the age of 70 it is just a tenth of the size in adolescents, meaning that the elderly become more vulnerable to infections and viruses such as flu.
Using the protein, dubbed FOXN1, the Edinburgh team was able to instruct immature cells in the thymus to regenerate the organ to more than twice the size than in the untreated mice.
Lead scientist Professor Clare Blackburn said: "By targeting a single protein, we have been able to almost completely reverse age-related shrinking of the thymus.
"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.
"However, before we test this in humans we need to carry out more work to make sure the process can be tightly controlled."
The ground-breaking study, which took five years to complete, used genetically modified mice that had been created with chemical "markers" that allowed scientists to manipulate their genes. Previous attempts to provoke thymus regeneration have been made using sex hormones, but these have resulted in only temporary recovery of size and function of the organ.
But the Edinburgh team is the first to prove that the organ can be regenerated by simply manipulating a single gene.
Dr Nicholas Bredenkamp, who also worked on the project, said: "By just manipulating a single gene we have been able to regenerate this organ, something that has never been done before.
"Now it remains to be seen if a similar technique can be used to regenerate other organs within the body."