Mucus-producing tissue lines the mouth, nose, sinuses, throat, lungs, and gastrointestinal tract. Mucus acts as a protective blanket over these surfaces, preventing the tissue underneath from drying out. "You have to keep them moist, otherwise they'll get dry and crack, and you'll have a chink in the armor," says Neil L. Kao, MD, associate professor of medicine at the University of South Carolina School of Medicine.
Mucus also acts as a sort of flypaper, trapping unwanted substances like bacteria and dust before they can get into the body -- particularly the sensitive airways. "You want to keep that environment, which is a sterile environment, free of gook," says Johns. "Mucus is kind of sticky and thick. It's got viscosity to it that will trap things."
But mucus is more than just sticky goo. It also contains antibodies that help the body recognize invaders like bacteria and viruses, enzymes that kill the invaders it traps, protein to make the mucus gooey and stringy and very inhospitable, and a variety of cells, among other things.