Mucus

Mucus is a slippery, viscous gel that originates from specialized cells within mucous glands and mixed glands containing both serous and mucous cells. It is a colloidal substance composed primarily of water (90–95%), along with inorganic salts, antimicrobial enzymes such as lysozymes, antibodies—particularly immunoglobulin A—and glycoproteins including lactoferrin and mucins. Mucins, produced by goblet cells in mucous membranes and submucosal glands, are large polymers that give mucus its characteristic rheologic or viscoelastic properties. The majority of mucus in the human body is generated within the gastrointestinal tract. Beyond humans, amphibians, fish, snails, slugs, and various invertebrates secrete external mucus from their epidermis to protect against pathogens, facilitate movement, and line fish gills. Plants and some microorganisms produce a similar substance known as mucilage.

Mucus forms a protective barrier over epithelial cells on both internal and external body surfaces, lining the respiratory, digestive, and urogenital systems, as well as the eyes and ears. It traps and removes inhaled particles—such as dust, pollutants, allergens, and infectious agents—while preventing the entry of pathogenic fungi, bacteria, and viruses. This barrier is selectively permeable, allowing the diffusion of small particles while blocking larger ones, a filtering process based on both particle size and surface chemistry. In the respiratory tract, mucus is part of the airway surface liquid, which includes a periciliary liquid layer surrounding cilia and an overlying gel layer. The continual movement of cilia toward the oropharynx, known as mucociliary clearance, helps transport mucus and trapped particles out of the lungs. Mucus also moisturizes inhaled air and prevents nasal and airway tissues from drying out. During illness, mucus may thicken with cell debris, bacteria, and inflammatory cells, becoming phlegm that can be coughed up as sputum. Conditions such as primary ciliary dyskinesia or cystic fibrosis—the latter caused by mutations in the CFTR gene—can disrupt mucus homeostasis, leading to accumulation, dehydration, and altered viscosity.