Mucus

Mucus (, MEW-kəs) is a slippery gel produced by, and covering, mucous membranes. Mucus forms a protective barrier between epithelial cells on the internal and external surfaces of the body and the outside environment. Mucus is found in the linings of the respiratory, digestive, and urogenital systems, and in the eyes and ears. Mucus helps to trap and remove inhaled particles, and prevent the entry of pathogenic fungi, bacteria and viruses at different sites. The mucosal barrier is nonetheless selectively permeable, allowing the diffusion of small particles and blocking the entrance of larger particles. Mucus is typically produced from cells found in mucous glands, and may also originate from mixed glands which contain both serous and mucous cells. It is a viscous colloidal gel containing water (90–95%), inorganic salts, antimicrobial enzymes (such as lysozymes), antibodies especially immunoglobulin A, and glycoproteins such as lactoferrin and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands. Most of the mucus in the body is produced in the gastrointestinal tract. Amphibians, fish, snails, slugs, and some other invertebrates also produce external mucus from their epidermis as protection against pathogens, to help in movement, and to line fish gills. Plants, and some microorganisms produce a similar substance called mucilage.