Scientists on Sunday said they had figured out how influenza viruses carried by birds latch on to humans, a discovery that may open the way to a vaccine against not just deadly avian flu but against all flu types. There are many strains of flu virus, but only a few have succeeded in crossing the species barrier from animals to humans. Strains known as H1 and H3 are the most common, and are especially efficient in attacking cells in the upper reaches of the respiratory system. Variants of the H5 virus, by contrast, usually remain confined to wild or domesticated fowl. But when they do infect humans it is often with lethal results, as immune systems are unable to recognize and counter the novel pathogen. Researchers have long known that whether an influenza strain infects humans depends on the ability of a protein on the surface of the virus, called hemagglutinin, to bind to a sugar receptor in the respiratory tract. But the study, led by Massachusetts Institute of Technology professor Ram Sasisekharan, says that the big factor is the shape of the sugar receptors in human lung cells. At the very least, the new discovery will help scientists rapidly identity strains that may develop the capacity to attack human respiratory systems.
