In this image, the community of cells lining the airways of a mouse is magnified more than 10,000 times. This collection of cells, known as the mucociliary escalator, is also found in humans. It's our first line of defense against inhaled bacteria, allergens, pollutants and debris. System malfunctions can cause or worsen lung infections and conditions such as asthma and chronic obstructive pulmonary disease. Cells shown in gray secrete mucus, which traps inhaled particles. The colored cells remove the mucus layer from the lungs.