Overview
Mucus (often called mucous or, informally, slime) is a slippery, gel-like secretion produced by many organisms. It acts as a biological lubricant that reduces friction and, in simple terms, plays roles similar to how oil works in machines. The term covers both thin surface coatings and thicker gels depending on where it is made and how it is used.
What it is made of. Mucus is dominated by large glycoproteins known as mucins and other long-chain polysaccharides that give it structure. Those chains are long and often described as long-chain polymers of carbohydrate-rich units; in other words they are complex carbohydrates with many sugar groups attached. In addition to mucins, mucus contains water, salts, lipids, shed cells, trapped particles and sometimes extracellular DNA from microbes or host cells.
Production and anatomy
In vertebrates mucus is produced at mucous membranes (mucosa) by specialised secretory cells. Individual goblet cells and multicellular glands deliver mucus to surfaces through ducts; in other words many mucus-producing tissues are exocrine glands that empty to a surface. The alimentary canal (the digestive tract), nasal passages, airways and sexual organs are prominent examples where mucus is continuously secreted to coat and protect surfaces.
Physical properties. Mucus behaves as a viscoelastic, gel-like material rather than a simple liquid. It is shear-thinning (becomes less viscous under force) and can trap particles while still allowing sliding or transport when moved by cilia or muscles. Those mechanical characteristics come from the mucin networks and their hydration state.
Functions and defence
Mucus fulfils several overlapping roles: lubrication, barrier protection, particle and microbe trapping, and a matrix for immune molecules. In the respiratory system mucus helps mucociliary clearance in the lungs, moving inhaled particles and microbes along with beating cilia while protecting underlying epithelial cells from damage. It contains antiseptic compounds and enzymes such as certain antimicrobial enzymes that degrade microbial components and lysozymes that attack bacterial cell walls. Mucus also carries immunoglobulins and antibodies that neutralise pathogens.
Diversity across species. Mucus is widespread: many mammals produce it internally, amphibians use external mucus to keep skin moist and help gas exchange across the epidermis in some cases, and fish coat themselves with mucus that protects gills and reduces infection. Gastropods such as snails and slugs produce external slime that aids locomotion and adhesion. Some unusual examples, like the hagfish slime, deploy a fibrous mucous network that rapidly expands to deter predators. Many invertebrates also secrete mucus for feeding, protection or communication. Even bacteria form extracellular polymeric substances that can function similarly to mucus in biofilms.
Medical and practical relevance
Changes in mucus quantity or quality are central to several health problems: excessive or thickened mucus contributes to chronic bronchitis, and abnormal mucus properties are a hallmark of cystic fibrosis where clearance is reduced. Mucus sampling is used diagnostically (sputum, cervical mucus) and its components inspire biomaterials research. In ecology and behaviour, mucus mediates reproduction (e.g., cervical mucus), feeding, and in some animals communication through chemical cues.
- Protects internal surfaces and traps microbes, including bacteria and other microbes.
- Contains enzymes such as lysozyme and antimicrobial factors that reduce infection risk.
- Forms part of mucociliary clearance in the respiratory tract and the lung system.
- Is produced by mucous membranes and exocrine glands throughout the body (e.g., digestive tract) including the alimentary canal and reproductive tract.
- Shows wide variation among vertebrates and mammals and plays special roles in amphibians and fish (gill protection).
- External slime in snails and slugs aids movement and hagfish produce defensive mucus.
- Mucus also interacts with sensory systems such as the auditory passages and eye surfaces to maintain function.
Because mucus sits at the interface between an organism and its environment, it is both a frontline defence and a medium for many biological processes. Understanding its chemistry, mechanics and variability is important in medicine, materials science and ecology. For further general overviews and resources follow introductory links and reviews on polymer properties and on mucin biochemistry.