They believe the discovery could pave the way for the development of better painkilling drugs.

The immune system, which acts as the body's defence mechanism fighting infection, was previously only associated with causing the inflammation linked with pain.

But US researchers have found that the immune system not only causes inflammation, but also tries to ease some of the pain it causes.

Dr Christoph Stein of Johns Hopkins University in Baltimore and the Berlin Free University, along with an international team of collaborators, found that certain white blood cells travel to the site of an inflammation and secrete natural painkillers known as beta-endorphins.

The cells use specialised proteins known as selectins to do this.

They tested their idea in rats and found that when they blocked selectin, the rats reacted more strongly to pain in swollen paws.

Writing in the journal Nature Medicine, Dr Stein's team the discovery may be used to design new types of painkillers.

"Our findings provide a foundation for the development of a new generation of analgesics," they wrote.

"Such new drugs would not have the side-effects of current opioid drugs, which affect the brain. The new drugs, like the natural substances, would act at the site of inflammation."

Selectins work by helping white cells to stick to the walls of the arteries they travel through, which they have to do before they can squeeze through the walls and make their way to the site of injury.

The discovery opens up the possibility of developing drugs which could imitate or assist selectins and so help more white cells to reach the scene of infection.

The scientists hope the effect will be better pain relief, without the side effects of internal bleeding, constipation and nausea that today's anti-inflammatory drugs often cause.

Exciting development

Current painkillers may be usurped

Describing the new research as "exciting", Frank Porecca, Professor of Pharmacology and Anaesthesiology at the University of Arizona, said: "Such new information is sure to provide fertile ground for exploration of novel approaches to pain control, including the development of analgesics that target the immune system and lack the side-effects of current pain killers.

"The advantage would be that it would be promoting the relief of pain right at the injury site, because the human body has already worked out a mechanism to allow these painkillers to accumulate at the injury site. So one could achieve local pain control without widespread side effects."

One problem will have to be overcome before they can be developed.

The white cells that congregate at some injuries do more harm than good, by themselves helping to cause painful inflammation.