Scientists have taken a step forward in working out what's required to make a vaccine to tackle the world's worst pandemic - AIDS. Dennis Burton and Anne Hessell, from the Scripps Institute in California, have shown that injecting an antibody called b12 can protect macaques against infection with the monkey equivalent of HIV. But when the team, who have described their findings in this week's Nature, modified certain parts of the antibody they found that despite still being able to recognise the virus, the antibody could no longer effectively protect experimental animals. The critical region seems to be the way the antibody recruits other components of the immune system, including white blood cells and a pathogen-disabling family of proteins called Complement. What this study shows is that it should be possible to produce a vaccine to stop HIV, but it will be critical to ensure that it drives the production of antibodies that can harness other arms of the immune system like the b12 antibody used in this study.