There are many strains of H.I.V., and no known antibody incapacitates all of them. But in the last few years, several teams of scientists have isolated about a dozen that each can shut down up to 80 percent of all virus strains. These are said to be “broadly neutralizing.”

Less than 20 percent of all patients naturally develop such antibodies in their blood, and even those who do aren’t fully protected. One of the women whose blood was crucial to Dr. Karim’s study has died of AIDS-related tuberculosis, and the other is on antiretroviral drugs.

Nonetheless, experts hope it will eventually be possible to manufacture cocktails with large doses of several kinds of antibodies to treat patients — or even to induce the immune system to make those particular antibodies, which would amount to a vaccine.

But that will take more work, and more luck.

Dr. John P. Moore, an AIDS researcher at Weill Cornell Medical School, called the South African paper “good solid science, but not enough to know if you have the right target.”

“It’s like looking at a castle and saying: ‘I can see a weak point, but I don’t know what kind of battering ram to get,’ ” he added.

Normally, H.I.V. repels antibodies by mutating its Velcro hooks into different shapes. But some spots on the viral shell don’t change shape easily. Scientists from the National Institute for Communicable Diseases in South Africa and universities in KwaZulu/Natal, Cape Town and North Carolina, as well as from Harvard, screened multiple blood samples looking for previously known antibodies. They found them in the two women, and noted how long into their infections those antibodies appeared — around six months, it turned out, after their infections were first detected.

Then the scientists looked to see what exactly had changed in the virus circulating in their blood at that time.