The normal function of cells that line the airways of the lungs is to produce mucus, which acts as a part of the body’s defense mechanism against bacteria, viruses, and other foreign particles. The mucus helps catch the particles while tiny hair-like projections in the airways called cilia clear the mucus out of the lungs. In chronic bronchitis, there is an overproduction of mucus that the cilia are unable to clear from the lungs. Over time, the cilia lose function and become less efficient at removing mucus from the lungs. This excess mucus results in airway narrowing and also provides a haven for bacteria to grow, thus making patients more susceptible to serious lung infections.