Scientists say they have made a landmark discovery which could pave the way for new drugs to beat illnesses like the common cold.

Until now experts had thought that antibodies could only tackle viral infections by blocking or attacking viruses outside cells.

But work done by the Medical Research Council shows antibodies can pass into cells and fight viruses from within.

PNAS journal said the finding held promise for a new antiviral drugs.

The Cambridge scientists stressed that it would take years of work and testing to find new therapies, and said that the pathway they had discovered would not work on all viruses.

The common cold

“Doctors have plenty of antibiotics to fight bacterial infections but few antiviral drugs. Although these are early days, and we don’t yet know whether all viruses are cleared by this mechanism, we are excited that our discoveries may open multiple avenues for developing new antiviral drugs,” said Dr Leo James at the Laboratory for Molecular Biology in Cambridge, who led the study.

The body tackles infections by unleashing biological foot soldiers called antibodies that stick to viruses as they circulate in the bloodstream. For the past 100 years, scientists working on immunity generally believed this made it harder for viruses to get inside healthy cells and so spread illness around the body.

But the new study has shown that for many viruses, antibodies work in a very different way. Instead of preventing viruses from infecting cells, the antibodies follow the invader inside and co-ordinate an immune attack from within.

Until now, the belief among immunologists was that antibodies went to work only outside cells. Once a virus had invaded a cell, it was thought to be too late for the immune system to do anything.

A virus is a microscopic bundle of genetic material that is wrapped in a protective protein coat. Viruses cannot multiply by themselves, but instead must hijack cells and replicate inside them. Scientists have identified millions of different viruses which infect humans, animals, plants and even bacteria.

The cold virus

In humans, viruses are responsible for influenza, the common cold, smallpox, chickenpox, shingles, herpes, polio, rabies, Ebola, hanta fever, Aids and various forms of cancer.

In a series of experiments, James’s group found that in many cases, antibodies do very little to stop viruses from infecting cells. Instead, the antibodies cling to the viruses when they invade cells and use the cells’ own biological machinery to kill the virus.

James showed that once inside an infected cell, antibodies attract a protein called TRIM21. This in turn signals to the cell’s equivalent of a waste disposal machine, a large cluster of proteins called a proteasome. When the proteasome arrives, it latches on to TRIM21 and goes to work, dismantling the virus piece by piece. The process happens quickly, and often before the virus has a chance to cause harm.

The discovery, which is reported in the journal Proceedings of the National Academy of Sciences, could pave the way for a new generation of antiviral drugs that fight infections by supercharging the body’s own defences.