Why does a nose run when you have a cold?

Why does nose run when have a cold?

• I get that it's fighting infection, but why so much mucus? How does mucus fight a cold/flu virus?

• Answer:

  The virus incites a lot of inflammatory changes in the mucus producing cells which line the nasal passages. When these cells become inflamed by any process, whether allergic or infectious, they produce abundant amounts of mucus since this is the only thing they can do. Part of the inflammatory process causes the small blood vessels in this region to dilate and fluid "leaks" out contributing to mucus formation. Once the agent which is causing the inflammation, either the virus or allergen is removed then the cells are no longer inflamed and there is no signal transduction mechanisms that produce the mucous-related proteins are down-regulated.