Helium ion micrograph showing cells lining the trachea of a mouse. These cells, also found in humans, are known as the mucociliary escalator. They are the first line of defence against inhaled bacteria, allergens, pollutants and debris. The grey cells shown secrete mucus to trap these particles and the coloured cells sweep the mucus layer out of the lungs. Cells viewed using a ZEISS ORION NanoFab microscope.