The body's own "training manual" for successfully attacking HIV has been recorded by scientists and it is hoped it can be used to design vaccines.
HIV mutates in order to survive the onslaught of a patient's immune system.
However, some patients develop highly effective antibodies that can neutralise huge swathes of HIV mutants.
An analysis of the arms race between body and virus, published in the journal Nature, has shown how these antibodies are made.
When someone is infected with HIV, their body produces antibodies to attack it. But the virus mutates and evades the offensive, so the body produces new antibodies that the virus then evades and the war goes on.
However, after about four years of this struggle some patients hit on to a winner by targeting something the virus finds harder to change - an Achilles heel.
"Even though the virus mutates and there are literally millions of quasi-species of virus because of all these mutations, but there are parts the virus can't change otherwise the virus cannot infect - these are the vulnerable sites," Prof Barton Haynes, of Duke University, in North Carolina, told the BBC.
At this stage of the infection it is far too late to make a difference for the patient as the virus is hiding in untouchable reservoirs.
However, some researchers believe that vaccines that encourage the body to produce these "broadly neutralising antibodies" may give people immunity to the virus.
The research team's study is based on a patient in Africa who had a rapid diagnosis, about four weeks after being infected with the virus.
They were eventually able to produce an antibody named CH103 that could neutralise 55% of HIV samples.
It was not produced in one easy step. Rather it was the product of the war of the immune system and HIV trying to out-evolve each other.
However, through regular genetic analyses of both the immune system and virus, researchers could piece together each of the steps that culminated in the production of CH103.
It is like a training manual for the immune system.
Prof Haynes said: "What we were able to do was map out the arms race of both virus and antibody, and in doing so we have now a map.
"This is the first time we've been able to see the actual road map."
He said the challenge now was to see if re-creating those steps could lead to a viable vaccine.
However, he said it would almost certainly need to be a vaccine combining multiple "Achilles heels" - in the same way that HIV therapies are a combination of drug treatments.
Prof Jane Anderson, consultant at Homerton hospital in London and chair of the British HIV Association, said: "The study gives important insights into the ways in which the human immune system responds to HIV infection and increases our understanding about the relationships between the virus and the human host.
"This is another welcome step on the path to develop vaccines against HIV."
Dr Sarah Joseph, who tests HIV vaccines at the Medical Research Council clinical trials unit, said: "This paper is really interesting. Some people do make antibodies that neutralise a lot of HIV virus, bit it is not of use to them as they produce it way too late."
She said harnessing these antibodies "could be a big deal" and there was "even talk about mass-producing antibodies and infusing people with them".