Mucus protects your respiratory system by lubricating and filtering the air you breathe. Mucus is produced by the mucous membranes that line your nose and lungs. Every time you breathe in, allergens, bacteria, viruses, dust, and other debris stick to the mucus and are then expelled from your respiratory tract. Sometimes, your body can produce too much mucus, forcing you to frequently clear it by coughing, spitting, etc.