Mucus

Mucus cells.

Mucus, 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 polysaccharides, which are long-chain carbohydrates. These molecules are stiff when dry, and sticky and slippery when wet. They are present in all types of organisms from bacteria to man.

In vertebrates mucus is made by covering mucous membranes. Mucus may have sickness-preventing enzymes (such as lysozymes) and immunoglobulins (antigens). Mucus protects epithelial cells in the lungs, gut, genital, visual, and auditory systems in mammals; the epidermis in amphibians; and the gills in fish. Snails, slugs, hagfish, and certain invertebrates also produce external mucus. As well as a protective function, slime helps movement and plays a role in communication.