Mucus: Composition, Production, and Protective Functions

Mucus is a viscoelastic gel lining all wet epithelia (respiratory, GI, urogenital, ocular). It is a critical barrier and homeostatic regulator.

Composition

• Water (~95%): Solvent and hydration.
• Mucins (glycoproteins): Large, heavily O-glycosylated proteins (MUC5AC, MUC5B, MUC2) – provide gel structure, ligand binding.
• Electrolytes, lipids, proteins: Antibodies (sIgA), lysozyme, lactoferrin, defensins, trefoil factors.
• Cellular debris, microbiota, inhaled/ingested particles.

Production

• Goblet cells: Secretory cells in epithelium (intestine, conjunctiva, airways).
• Submucosal glands: Serous/mucous acini (trachea, bronchi).
• Stimulated by irritants, pathogens, cytokines, neural signals.

Functions

• Barrier: Physical trap for pathogens, particles, allergens.
• Clearance: Mucociliary escalator (respiratory); peristalsis (GI).
• Lubrication/Protection: Prevents epithelial desiccation, shear stress, acid/enzyme damage.
• Immune Interface: sIgA neutralization; antimicrobial peptides; microbiome habitat.
• Signaling: Mucin glycosylation patterns modulate immune responses.

Clinical Relevance

• Hypersecretion: Asthma, COPD, CF, bronchitis.
• Hyposecretion/Altered quality: Dry eye, Sjogren's, atrophic gastritis.
• Cystic Fibrosis: CFTR defect → dehydrated, viscous mucus → plugging, infection.