An antibody could hold the key to stamping out HIV - the virus which has infected more than 36 million people worldwide.

The rare human antibody, known as B12, is capable of deactivating different strains of HIV which can lead to Aids.

Scientists at Oxford University are working with colleagues at The Scripps Research Institute in California, USA, to study the molecule and work out how it could be used to form new HIV vaccines.

Dr Pauline Rudd, a reader in Glycobiology at the university's biochemistry department, said the antibody was first discovered after researchers looked at HIV victims who had been diagnosed for many years without becoming ill.

B12 was found in the bone marrow of a 31-year-old American man but, until recently, scientists had not been able to see how it fought the HIV virus.

Dr Rudd said: "The importance is that because we now understand how it works, we can try to produce the antibody ourselves.

"It gives us a lead to producing a vaccine.

"There's an awful long way to go, but B12 is the template we need to design antibodies."

HIV damages the human immune system by sticking to the body's natural defence cells. As a result, patients become vulnerable to many different infections that healthy people can ward off.

It can develop into Aids, which has killed 2.8 million people worldwide.

The B12 pictures have been published today in Science, as part of a paper by Erica Ollman Saphire, of The Scripps Research Institute.

They show that B12 has a long finger-like arm which penetrates and neutralises the HIV virus, leaving it unable to invade the immune system.

Previous vaccine studies have been hindered because there are many sub-types of HIV. But B12 appears to be effective against a wide variety of these different forms.

The scientists are now studying how the special antibody is triggered in the body.