DeVico's idea was to build a vaccine consisting of gp120 permanently fused to its CD4 receptor.

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But gp120's innards are exposed for 30 minutes or so while the protein is hooked to its human receptor, called CD4, in preparation to invade a cell.

FORBES: Outsmarting AIDS

Prof Jonathan Weber, from Imperial College London, said those on the 48-week regime "end up with much higher CD4 cell count and a much lower viral load".

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The researchers used a sophisticated statistical modeling technique to assess viral load and CD4 cell loss in more than 2, 800 patients with HIV who were not receiving treatment.

BBC: Cell infected with HIV

Those taking the drug also had a higher CD4 count.

BBC: Cell infected with HIV

Eventually, he began to study an antibody called Ibalizumab that was designed to block HIV from attaching to CD4, the primary receptor used by the virus to infect a cell.

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This consists of two sugar-protein molecules known as gp120 and gp41 that react with a protein called CD4, which is found on the surface of certain cells of the immune system.

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Two specific types of immune system cell that attack invaders - CD4 T-cells and natural killer cells - increased in number with age, with a higher rate of increase in women than in men.

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Edwin Bernard, editor of AIDS Treatment Update, said it was known that CD4 decline varied enormously between people with a similar viral load, and that treatment guidelines had begun to play down the importance of the measure as a diagnostic tool.

BBC: Cell infected with HIV