A moist tissue lining the body’s internal passages and cavities that open to the outside, including the mouth, nose, throat, stomach, intestines, and airways. Also called mucosa, it secretes mucus, a slippery fluid that lubricates, traps particles and microbes, and protects underlying tissues from acid, enzymes, and abrasion. In the gut, the mucous membrane also absorbs nutrients; in the airways, it humidifies incoming air. Its integrity is vital: damage from infection, inflammation, or dryness impairs these defences. Vaccines delivered nasally or orally exploit mucosal immunity, and nutrition supports mucosal health through vitamin A and adequate protein.