Why did H1N1 cause some to develop pneumonia and die while leaving most with only mild flu symptoms?

The answer may lie in the elevated presence of a single molecule in those who suffered most from the virus, according to international research led by Toronto scientists.

“We’ve been doing this for many years and this is the first real clue we’ve had between severe and mild illness,” says Dr. David Kelvin, head of experimental therapeutics at Toronto’s University Health Network.

“It’s the first thing we can put our finger on and actually say ‘This is an area (where) we should intensify our research efforts,’” said Kelvin, the senior study author.

The molecule, known as interleukin 17, may be the culprit in causing severe symptoms in a host of seasonal influenzas and other respiratory ailments like SARS, he said.

The study appears this week in the journal Critical Care.

Looking at a group of otherwise healthy Spanish patients during H1N1’s initial May appearance, the study found that those with severe symptoms also had elevated levels of interleukin 17, compared to those with mild ailments.

Known as a cytokine, the molecule is one of several that help regulate our body’s immune response to viruses and bacteria. But interleukin 17 is also associated with inflammatory autoimmune diseases such as rheumatoid arthritis and asthma, Kelvin said.

It is this inflammatory function that is likely at play in those flu sufferers with elevated levels of the substance, he says.

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