Surfactant protein A, illustration. This is one of the components of pulmonary surfactant, the lipoprotein complex that is an essential component of a healthily functioning lung. Surfactant proteins also play a role in the immune system. This protein takes the form of a monomeric unit, with four regions labelled here: the carbohydrate-recognition domain, the neck region, the collagen-like region, and the n-terminal segment.