Mucus is a thick, slippery and usually clear substance that is produced in the lining of body cavities or canals, such as the nose, throat, sinuses, and airways of the lungs. Mucus protects and moistens the lining of body organs, such as the stomach and intestines, and traps dust particles, smoke, bacteria, and other irritants that enter the nose when a person inhales.
Mucus that becomes too thick and sticky can block the airway, digestive system, or cause other problems in the body. Thick and sticky mucus is difficult for the body to remove and can increase the risk of infection.
WebMD Medical Reference from Healthwise