Scientists have shown what happens when an infection-fighting antibody attacks a gap in HIV's formidable defences.
Antibody (green) locks onto a key site on the virus
The National Institute of Allergy and Infectious Diseases-led team say the work could aid HIV vaccine development.
They have published an atomic-level image in Nature showing the antibody, b12, attacking part of a protein on surface of the virus.
HIV avoids attack by constantly mutating, but this protein segment is a weak spot because it remains stable.
Dr Elias Zerhouni, director of the US National Institutes of Health (NIH), said: "Creating an HIV vaccine is one of the great scientific challenges of our time.
"NIH researchers and their colleagues have revealed a gap in HIV's armour and have thereby opened a new avenue to meeting that challenge."
Developing a vaccine for HIV has proved extremely difficult.
The virus is able to mutate rapidly to avoid detection by the immune system, and is also swathed by a near-impenetrable cloak of sugary molecules which block access by antibodies.
But certain parts of the virus must remain relatively unchanged so that it can continue to bind to and enter human cells.
A protein, gp120, that juts out from the surface of the virus and binds to receptors on host cells, is one such region, making it a target for vaccine development.
Previous analysis of the blood of people who have been able to hold HIV at bay for long periods has revealed a rare group of antibodies - including b12 - that seem to fight HIV with a degree of success.
The latest study has revealed the detailed structure of the complex, which is formed when b12 docks with gp120.
Until now this has proved impossible, because of the flexible nature of some of the chemical bonds involved.
But the NIAID team were able to stiffen up the key protein enough to capture a picture of the complex.
They hope that revealing the structure of this bond in such precise detail will provide clues about how best to attack HIV.
Researcher Dr Gary Nabel said the work had revealed a "critical area of vulnerability on the virus".
He said: "This is certainly one of the best leads to come along in recent years."
Keith Alcorn, editor of the aidsmap.com website, said vaccines based on antibodies had so far failed to produce promising results.
"These findings are very important because they show what sort of antibodies are likely to be most successful in neutralising HIV.
"Now the challenge is to develop vaccine products that can be tested in humans."