AIDS drug cocktails may be able to restore the ravaged immune systems of some people infected with HIV, researchers reported on Wednesday.

Immune cells known as CD4 T-cells returned to normal levels in an ideal group of patients, picked because they responded optimally to a combination of at least three AIDS drugs, the researchers reported in the Lancet medical journal.

The human immunodeficiency virus, which causes AIDS, plunders the immune system, leaving people vulnerable to a range of infections that may prove fatal.

AIDS is incurable, but doctors try to prop up the immune system with life-extending drug therapy aimed at reducing the amount of virus in the body.

The study involved 1,835 HIV-infected people drawn from a larger study involving more than 14,000 patients from across Europe, Israel and Argentina.

"I think it's very encouraging that if people can respond to treatment well enough and can suppress the virus for long enough, we have sufficient evidence to say their CD4 counts can return to normal," Dr. Amanda Mocroft of Royal Free and University College Medical School in London, one of the researchers, said in a telephone interview.

"Our previous understanding was that there was a plateau in CD4 counts so that CD4 counts would stop increasing after a sufficiently long time taking combination therapy," she added.

Mocroft said not all HIV patients respond as well to these drugs, and many, particularly in the hardest hit regions like sub-Saharan Africa, do not have access to them.

"This is sort of the best-case scenario, if you like, that we can identify a group of patients who we would expect to have a normal CD4 count with sufficient treatment," Mocroft said.

These patients were chosen because they responded well to the treatment, with the drugs suppressing the virus to very low levels. They were tracked for about five years.

Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said doctors who care for HIV-infected patients have noticed this restoration of normal levels of CD4 cells in some of them. Fauci credited Mocroft's team for documenting this phenomenon in a systematic way.

CD4 cells, a type of white blood cell, help protect the body from infection. But HIV targets CD4 cells, using them to create more copies of the virus, thus undermining the immune system.

After initial infection, a person can produce more CD4 cells to take the place of those attacked by HIV. But in time, the body cannot make enough, increasingly weakening the immune system.

Although it is impossible to eradicate the virus with existing drugs, it is possible to keep it at extremely low levels in some people with the right combination of drugs.

The AIDS virus infects close to 40 million people globally, most of them in Africa. It has killed more than 25 million.