In a dramatic breakthrough that rewrites the medical textbooks, scientists have discovered a new immune defense, which could cure the common cold.

Researchers in Cambridge, England, found that viruses can be destroyed by the immune system even after they have invaded human cells.

Previously it had been thought that antibodies could only kill viruses outside the cell.

The discovery raises the possibility of highly effective drugs against the common cold, the winter vomiting virus and rotavirus, which causes severe diarrhea.

Scientists funded by the Medical Research Council found that antibodies are able to cling on to viruses as they invade human cells. A virus would then normally take over the cell's machinery to make more copies of itself.

But the presence of the antibody inside the cell triggers a mechanism that kills the virus within two hours — long before the virus is able to do any damage.

The scientists now hope to find drugs - perhaps a nasal spray or inhaler - that could enhance the virus-destroying mechanism inside the cell.

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