A group of researchers believe they have discovered why some people are more resistant to HIV, the virus that causes Aids.

The scientists, at the National Centre of Biotechnology in Madrid, have established how molecules can group together to stop the virus from infecting cells.

They say their work could lead to the production of a new type of drug against HIV.

Mutant receptor

HIV infects white blood cells by exploiting a set of molecules that sit on the cells' surface.

These receptor molecules bind with a group of chemicals called chemokines which activate the immune system to protect the body against invaders.

But they can also bind with HIV and this is how the virus infects the cells.

Scientists in Madrid have discovered that people with a mutant form of one type of receptor molecule are able to resist HIV for longer than those without it.

They say the mutant CCR2 receptor acts like a key that locks the door and prevents the virus from entering healthy cells.

The researchers have published their work in the journal Nature. They say that now they know how the mutant receptor works they hope drug companies will use the information to start producing new types of drugs against HIV.

Current drugs which delay the progress of the disease either stop HIV-infected cells from maturing or stop the virus from reproducing.

The pharmaceutical company, Glaxo Wellcome, which is carrying out research into similar types of drug, says the results will add to understanding of HIV infection.

But it will take a long time and much more work before any drugs can be produced.