A critical function of mucin in saliva is to protect the mouth from drying out. Mucin also lubricates the tissues and teeth in the mouth and protects them against pathogens and toxins.
Salivary mucin also plays a role in the protection of tooth enamel. Mucin clings to the crystals of hydroxyapatite on the enamel and gives it a protective layer called pellicle.
There are two forms of salivary mucins: high or low molecular weight mucins. Low molecular weight mucin is found more often in saliva than high molecular weight mucin. It is also abundant in people who are resistant to cavities, and can even change high molecular weight mucin into the lower molecular form. Salivary mucins are also crucial to the balance of calcium in the mucous membranes.
Mucin is a glycosated protein, which means that it is coated with carbohydrates. This coating allows mucin to hold on to water and protects protein from being broken down. When a person chews food, saliva in the mouth softens it and threads of mucin and quantities of water hold it together in a bolus. This makes the food easy to swallow and protects the digestive tract from being damaged or scratched by the food.