The antibody, taken from an unusual patient whose body can resist the virus, recognizes and attacks the human immunodeficiency virus, unlike most of the body's defenses.

"Nothing like this has ever been seen before," Ian Wilson of The Scripps Research Institute in La Jolla, California, who led the research, said in a statement.

AIDS has killed 25 million people around the world and is projected to kill 80 million by 2010. The only real hope of fighting the incurable virus is a vaccine, but efforts so far have flopped although dozens of vaccines are being tested.

Antibodies are an important arm of the body's defenses against germs. They are usually able to recognize an invader by structures on its surface, called antigens, and can either call in help, or neutralize the invader themselves by pasting themselves against it.

Most vaccines in use today stimulate the production of neutralizing antibodies.

The human body makes plenty of antibodies against HIV (news - web sites), but the virus disguises itself with human sugars.

One antibody seems to be able to see past this ruse. Called 2G12, it was found by Austrian researchers a decade ago in a patient who seemed to resist AIDS -- the condition caused as HIV destroys the immune system over time.

Writing in the journal Science, Wilson and colleagues said they had figured out how 2G12 does it.

It recognizes that while HIV is covered up with human sugars, they are not arranged in a human-like way.

The antibody does this with a special structure of its own, which Wilson and colleagues, including a team at Oxford University in Britain, have crystallized and imaged.

"The Fab (antigen-recognition) arms are interlocked," said Scripps researcher Dennis Burton, who worked on the study. "That is a unique arrangement, and it is good for recognizing a cluster of shapes like sugars on a virus."

Now what needs to be done is to use the structure of the antibody as a template to design an antigen to stimulate the production of 2G12 or another antibody that will neutralize HIV, the researchers said.

The approach might also work for making vaccines against other germs, said Wilson.

"Can we now use this to engineer antibodies with higher affinity against other antigens or clusters of antigens?" he asked.