The pulmonary collectins, surfactant proteins A1, A2, A3 and D (SFTPAs, D), play important roles in innate host defense by binding and clearing invading microbes from the lung. They also influence surfactant homeostasis, contributing to the physical structures of lipids in the alveoli and to the regulation of surfactant function and metabolism. They are directly secreted from alveolar type II cells into the airway to function as part of the surfactant. The mechanism of secretion is unknown. Mutations in SFTPA2 disrupt protein structure and the defective protein is retained in the ER membrane (thus not secreted). Lack of SFTPA2 in surfactant contributes towards idiopathic pulmonary fibrosis (IPF; MIM:178500) (Wang et al. 2009). The mechanism of pathophysiology is unknown.