Mucous membranes refer to a thin, moist lining that covers various internal organs and body cavities. These membranes are composed mainly of mucin-producing cells, which secrete a viscous fluid that helps protect the underlying tissues from infection and other harmful substances. Mucous membranes play an important role in filtering out foreign particles and pathogens through their surface follicles and goblet cells, which produce mucus to trap and neutralize these substances. Some of the most well-known mucous membranes include those found in the nose, mouth, throat, lungs, stomach, intestines, and reproductive organs such as the cervix.