A drug which could prove a safer way of protecting transplanted organs from rejection was inspired by children with a rare immune defect.
Transplant rejection is a major issue
Patients with X-Scid have to live in sterile conditions, and without treatment, often die young.
US researchers tailored a drug to produce some of the characteristics of X-Scid in transplant patients.
Writing in the journal Science, they say their drug prevents rejection, but without life-threatening side-effects.
Transplanted organs are under threat because the body's own immune system identifies them as foreign intruders and launches an attack which destroys them.
To stop this happening, transplant patients take drugs which suppress their immune systems, but in the long-term, these raise the risk of certain types of cancer, diabetes and high blood pressure.
The team, from Stanford University, tested their new drug on monkeys given transplant organs, and found it works in low doses, and does not appear to threaten long-term health problems.
This is because it directly targets a small key area of the immune system rather than suppressing immunity on a wide scale.
The idea for this came from X-Scid patients, who have dysfunctional immune cells that render them almost completely unable to fight off infections.
The condition can only be corrected by a bone marrow transplant or pioneering gene therapy.
The researchers wanted to harness some of the features of X-Scid without completely wrecking the immune system.
X-Scid is caused by mutations on a single gene vital for the production of a chemical called janus kinase 3, which sends a signal to activate immune cells.
They found a drug which is able to block the activity of this chemical in the body, thus suppressing the immune system cells precisely rather than huge numbers of other cells, as current anti-rejection medications do.
Dr Dominic Borie, who led the research, said: "We can say that it does as well if not better than then other immunosuppressive drugs that we have now.
"The encouraging thing is that we haven't seen any of the major problems that we could have seen.
"If we verify in humans that the side effects are minimal, then it may turn out that the drug could be used to treat a variety of autoimmune diseases in which the immune system needs to be controlled, and not just organ transplant recipients."
Tests have begun to see if it has an impact on psoriasis, a skin condition in which cells grow far more quickly than normal.