What is Mucus?
[[Image:Mucus cells.png|thumb|Mucus cells]] '''Mucus''' (mucous), or slime, is a slippery liquid. It is made by many living things as a kind of biological [[lubricant]]. It plays the role in living things which [[oil]] plays in machines. Mucus is mainly composed of [[polysaccharide]]s, which are [[polymer|long-chain]] [[carbohydrate]]s. These molecules are stiff when dry, and sticky and slippery when wet. They are present in all types of organisms from [[bacteria]] to humans. In [[vertebrate]]s mucus is made by [[mucous membrane]]s. Mucus may have [[antiseptic]] [[enzyme]]s (such as [[lysozyme]]s) and [[immunoglobulin]]s ([[antigen]]s).<ref>Singh P.K. ''et al'' 2002. A component of innate immunity prevents bacterial biofilm development. ''Nature'' '''417''' (6888): 552–5. [http://www.nature.com/nature/journal/v417/n6888/full/417552a.html]</ref> A major function of this mucus is to protect against infection by [[fungi]], [[bacteria]] and [[viruses]]. Mucus protects [[Epithelium|epithelial cells]] in the [[lung]]s, [[gut]], [[genital]], visual, and [[hearing|auditory]] systems in [[mammal]]s; the [[epidermis]] in [[amphibian]]s; and the [[gill]]s in [[fish]]. [[Snail]]s, [[slug]]s, [[hagfish]], and certain [[invertebrate]]s also produce external mucus. As well as a protective function, slime helps movement and plays a role in communication. Mucus is produced in many parts of the body. All the [[alimentary canal]], nose, and sexual organs have mucus glands which pass the mucus to the surfaces. The surfaces are mostly internal surfaces, though in some animals mucus covers the outside of the animal. [[Slug]]s are an example of this. Mucus glands are [[exocrine gland]]s, so they pass their mucus to the surface along [[wikt:duct|duct]]s. == Alkaline mucus == In the human [[digestive system]], mucus is used as a lubricant for materials that must pass over [[membranes]], such as food passing down the [[oesophagus]]. A layer of mucus along the inner walls of the [[stomach]] is vital to protect the cell linings