A compound found naturally in blood could form the basis for an entirely new class of Aids drugs, according to scientists. The chemical prevents the HIV virus from entering human immune cells and it is effective even against strains of the virus that are resistant to other drugs.

"You want a lot of drug classes, because multi-drug resistant viruses are starting to show up more and more," said Frank Kirchhoff at the University of Ulm in Germany, one of the authors of the study.

The newly identified compound prevents the virus from attaching a molecular anchor to the cell it is invading. No existing drugs affect this stage of infection, so the team hopes the compound could be modified to form a new class of similar drugs. With nearly 40 million people living with HIV worldwide and 3m deaths last year, new approaches are urgently needed.

The team discovered the compound - called Virus-Inhibitory Peptide or VIRIP - by screening hundreds of proteins from human blood. "A number of studies have suggested that some compounds in the human blood are able to inhibit HIV-1 and control it," said Prof Kirchhoff. One way was by adding proteins to cells that fluoresce when the virus is active. If the protein turned the lights off they knew it might be stopping HIV. The results are reported in the journal Cell.

VIRIP is a fragment of a larger protein, but the team is not sure whether it has a function itself. Nor do the researchers know yet exactly how it inhibits HIV. They do know its precise sequence is crucial: adding or subtracting just one of its 20 protein building blocks destroys its ability.