Scientists say they have learnt how the body controls the machinery it uses to fight infections and foreign invaders.
The immune system constantly combats bacteria and viruses
The advance, published in the journal Nature, may one day help find ways to tackle unwanted immune reactions following transplant surgery.
The Johns Hopkins University researchers say a protein molecule called carabin may be the body's way of restraining its defences.
The US-based team describes it as a "built-in timer" for the immune system.
Immunity is vital to human survival - the body is constantly confronted with things that should not be there, including bacteria and viruses.
The system is constantly adapting when faced with new threats from unknown organisms.
However, an over-powerful or runaway immune response can be a disadvantage too, as some illnesses involve the immune system attacking parts of our own bodies after failing to recognise them.
The Johns Hopkins team, led by Professor Jun Liu, has been hunting for body chemicals that might shed light on how the immune system is controlled.
They found that a protein called carabin appeared to be important, latching onto microscopic cells active during an infection.
It is made by white blood cells, one of the most important immune system cells.
However, its role actually appears to restrict their ability to mount a response to infection.
They found that when there was more carabin in a cell, it appeared to "damp down" its activity.
Professor Liu said: "It acts like an internal brake to dial down the speed and intensity of an immune response so that it doesn't go too fast or too far, or career out of control and attack healthy cells.
"It's like having a built-in timer to keep the immune system in check."
Carabin appears to work in a similar manner to drugs such as cyclosporin, which are used to control rejection of transplanted organs.
Professor Liu suggested that one eventual use might be a new drug to do this, and perhaps also help control "auto-immune" illnesses such as multiple sclerosis.
Dr Peter Peachell, a pharmacologist from Sheffield University, UK, said that the paper offered "interesting possibilities".
But he cautioned that the practical difficulties of developing and manufacturing a drug incorporating a large molecule such as carabin could be substantial.
He said: "Carabin appears to act on the same target as immune suppressing drugs such as cyclosporin, so offers a potential alternative.
"What this research does is tell us more about how the immune system is regulated, so it brings the possibility of more effective treatments for a variety of autoimmune diseases, although this is certainly some way off."
He said that in some circumstances, there might be benefits to switching carabin off, not increasing it - such as in the early stages of infection by viruses such as HIV, when a robust and sustained immune response might be useful.