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Flu’s
little helper
May/June 2010
by Margot
Sanger-Katz ’02
Influenza
is a nasty bug on its own, but as doctors have long known, the other infections
flu patients pick up while sick are often what kill them. This was the pattern
during last year’s swine flu pandemic: some fatalities were caused by flu
alone, but more were caused by the combination of influenza and bacterial
pneumonia.
Immunobiologist
Ruslan Medzhitov wanted to find out why flu patients so often succumb to common
bacteria that our immune systems can usually dispatch. His lab studied flu
infection in mice. When he injected mice with a common bacterium that “you get
every time you eat a chicken sandwich,” mice that already had the flu got much
sicker than healthy mice. By measuring several key immune system markers,
Medzhitov found that the immune systems of flu-infected mice were significantly
compromised, and that the effect lasted for about five days. Further research
revealed the cause: flu infection appeared to trigger production of an immune-suppressing
hormone.
But
if a disabled immune system made the mice susceptible to bacterial infections,
a fully active one had worse effects. When Medzhitov’s team repeated their
experiments with surgically altered mice unable to produce the hormone, mice
died at even higher rates. Unchecked, their immune response to flu was so
aggressive that it wreaked havoc on their lungs, destroying critical tissue.
Medzhitov concluded that the hormone’s immune-dampening effect, though
dangerous, is essential for surviving influenza.
The
study, published in Cell
Host and Microbe, doesn’t provide any medical advice. But it does confirm the folk wisdom that
when you get the flu, it’s good to stay home for a few days and keep yourself
safe from other germs. “Grandmother was right,” says Medzhitov. 
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