Phlegm comes from the respiratory system, particularly the lungs and throat. It is a specific type of mucus that is thicker than the mucus produced in the nose. Phlegm is produced by secretory cells such as goblet cells and submucosal glands in the airways to trap and clear irritants, bacteria, viruses, and other debris from the respiratory tract. It helps protect the lungs and airways from infection and inflammation. Normally, the body produces about 1 liter of mucus daily, which helps keep tissues moist and traps unwanted particles. When the respiratory system is irritated by infections, allergies, pollutants, or other factors, it produces more mucus—phlegm—to help expel these substances, often by coughing. Phlegm then either drains down the throat to be swallowed or can be coughed up and expelled from the body.