A previously unknown natural defence against swine flu and other viruses has been discovered, which could lead to new treatments.
The virus-fighting proteins were identified 25 years ago but their role has been a mystery until now.
Scientists found that knocking the protein IFITM3 out of human cells allowed the H1N1 swine-flu virus to run wild, but when their levels were artificially increased, replication of the viruses was completely blocked.
Professor Stephen Elledge, from Harvard Medical School in Boston, USA, who led the research, said: "We've uncovered the first-line defence in how our bodies fight the flu virus.
"The protein is there to stop the flu. Every cell has a constitutive immune response that is ready for the virus. If we get rid of that, the virus has a heyday."
The proteins are believed to redirect the traffic of viral particles entering cells and route them to a disposal area where they can be degraded and rendered harmless.