During cold season, the slimy snot filling your nostrils can make you feel like a mucus pump.
Now scientists have discovered how the body's infection-fighting machinery shuts down the sniffles and congestion.
The finding helps to solve a mystery of why the common cold — which can seem to go on forever — lasts only so long.
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The new study, detailed this week in the online edition of the journal Nature, revealed that a key protein called Carabin is responsible for curbing the body's immune response to viruses like those that cause the common cold.
"It's like having a built-in timer to keep the immune system in check," said lead researcher Jun Liu of Johns Hopkins University.
A case of the sniffles is one sign that your body is trying to rid itself of a viral invader.
In order to survive, a virus needs to find living cells to invade. Once inside your body, the virus hijacks some of the body's cells, turning them into factories that churn out viral particles en masse.
To push out the intruders, your immune system's white blood cells attack the virus. Extra mucus is created to clear viral particles from your nasal passages.
If the immune response were to continue indefinitely, the body could become a battleground where healthy cells would be killed, as is the case with autoimmune diseases.
After identifying Carabin as active during an infection, Liu and his colleagues added the protein to white blood cells already in attack mode. The cells became wimpy fighters as more Carabin was added.
With other lab tests, the scientists discovered a complex chemical conversation between Carabin and two enzymes.
One in particular, called Calcineurin, is critical for whipping white blood cells into a frenzied attack mode and also for boosting snot flow from your nostrils.
Turns out, this enzyme also causes the Carabin protein — the sniffles brake — to ramp up.
As is the case with many protein systems in the body, the production of one protein can trigger the production of others.
"The signaling enzyme Calcineurin activates the production of Carabin, which in turn inhibits Calcineurin itself," Liu told LiveScience.
If further research shows Carabin to be a key inhibitor of immune responses, it could be used to help prevent the rejection of a transplanted organ — which can be thought of as the artificial equivalent of a viral invader.
As for stopping the sniffles, Liu said: "We do not have direct evidence that this would be directly responsible for the duration or length of people's responses to a cold. But it is imaginable that the level of this protein could dictate that to a certain extent. If you don't have enough [Carabin], the symptoms could last longer."
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