Club cell

The club cell, formerly called the Clara cell, lives at the end of the respiratory tree, in the bronchioles, the finest tubes that air passes through. It has no cilia, the tiny hairs that sweep mucus along in the larger tubes: it is a nonciliated cell, cuboidal or low columnar in shape, with secretory granules stored inside.

It does three different things. It secretes part of the surfactant, the substance that coats the walls of the airway and keeps them from sticking to each other. It breaks down toxins, meaning it dismantles harmful substances that arrived with the air. And it serves as the progenitor cell for that area, which means it can divide and replace cells when the lung is injured.

The surfactant it makes has a very specific effect: it helps the bronchiole open as air comes in and keeps it from closing all the way as air goes out. It works in the narrowest stretch of the respiratory tree, and the oily film it makes is what keeps the walls of such a fine tube from sticking together as they empty.