Mucus
It is a viscous colloid containing inorganic salts, antimicrobial enzymes (such as lysozymes), immunoglobulins (especially IgA), and glycoproteins such as lactoferrin[1] and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands.
[4][5][6] The periciliary liquid layer surrounding the cilia consists of a gel meshwork of cell-tethered mucins and polysaccharides.
[7] The mucus blanket aids in the protection of the lungs by trapping foreign particles before they can enter them, in particular through the nose during normal breathing.
[8] Mucus is made up of a fluid component of around 95% water, the mucin secretions from the goblet cells, and the submucosal glands (2–3% glycoproteins), proteoglycans (0.1–0.5%), lipids (0.3–0.5%), proteins, and DNA.
[7][4] MUC5AC is the main gel-forming mucin secreted by goblet cells, in the form of threads and thin sheets.
Small particles such as dust, particulate pollutants, and allergens, as well as infectious agents and bacteria are caught in the viscous nasal or airway mucus and prevented from entering the system.
This process, together with the continual movement of the cilia on the respiratory epithelium toward the oropharynx (mucociliary clearance), helps prevent foreign objects from entering the lungs during breathing.
In addition, mucus aids in moisturizing the inhaled air and prevents tissues such as the nasal and airway epithelia from drying out.
Mucociliary action carries it down from the nasal passages and up from the rest of the tract to the pharynx, with most of it being swallowed subconsciously.
Excess nasal mucus, as with a cold or allergies, due to vascular engorgement associated with vasodilation and increased capillary permeability caused by histamines,[14] may be treated cautiously with decongestant medications.
Thickening of mucus as a "rebound" effect following overuse of decongestants may produce nasal or sinus drainage problems and circumstances that promote infection.
[15] In the lower respiratory tract impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may result in mucus accumulation in the bronchi.
[16] The dysregulation of mucus homeostasis is the fundamental characteristic of cystic fibrosis, an inherited disease caused by mutations in the CFTR gene, which encodes a chloride channel.
[22] In the human female reproductive system, cervical mucus prevents infection and provides lubrication during sexual intercourse.
[25] The green color of mucus comes from the heme group in the iron-containing enzyme myeloperoxidase secreted by white blood cells as a cytotoxic defense during a respiratory burst.
In the case of bacterial infection, the bacterium becomes trapped in already-clogged sinuses, breeding in the moist, nutrient-rich environment.
As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green.
The swelling capacity of mucus stems from the bottlebrush structure[31] of mucin within which hydrophilic segments provide a large surface area for water absorption.
[34] The high selective permeability of mucus plays a crucial role in the healthy state of human beings by limiting the penetration of molecules, nutrients, pathogens, and drugs.
