type II pneumocyte

The type II pneumocyte is the alveolus’s small but indispensable caretaker. While the flat type I cells form most of the wafer-thin wall where gases cross, these plumper cells, tucked into the corners, keep the air sac working and able to heal.

Their headline job is making surfactant — a soapy mixture that coats the inside of each alveolus and lowers surface tension, stopping the tiny sacs from collapsing as you breathe out. They are also the lung’s repair crew: when type I cells are injured, type II cells multiply and can turn into new type I cells to rebuild the lining.

Their importance shows up at the extremes of life and disease. A surfactant shortage in premature babies causes neonatal respiratory distress, and injury to these cells contributes to ARDS in adults; they are also a cell type targeted by certain respiratory viruses.