BEIJING, Jan. 7 (Xinhuanet) -- U.S. researchers found that flu viruses must be able to pick a very specific type of lock before entering human respiratory cells, media reports quoted researchers as saying Monday.

    Shape difference may explain why humans can get bird flu from a bird and not pass it along easily to other humans, said Ram Sasisekharan, a professor of biological engineering and health sciences at Massachusetts Institute of Technology in Cambridge.

    The researchers found that the most infectious human flu viruses bind with the umbrella-shaped receptors in the upper respiratory tract. They believe the H5N1 bird flu virus would need to adapt so it could latch on to these umbrella-shaped receptors before it could be spread readily from human to human.

    Before a flu virus can enter a human respiratory cell, a protein on the surface of the virus must bind with glycans that sit on the outside of the cells. In birds, the virus binds with alpha 2-3 receptors; in humans, it binds with alpha 2-6 receptors, they added.

    To infect humans, the H5N1 bird flu virus would need to simply mutate so it could bind with alpha 2-6 receptors. But it turns out not all alpha 2-6 receptors are the same. Some are short and cone-shaped and some are long and umbrella-shaped, said Sasisekharan.

    The discovery may help scientists better monitor changes in the H5N1 bird flu virus that could trigger a deadly pandemic in humans. And it may lead to the development of new and better drugs to treat flu viruses.

    (Agencies)