Surfactant protein A is an
innate immune system collectin. It is water-soluble and has collagen-like domains similar to
SP-D. It is part of the innate immune system and is used to opsonize bacterial cells in the
alveoli marking them for
phagocytosis by alveolar
macrophages. SP-A may also play a role in negative feedback limiting the secretion of pulmonary surfactant. SP-A is not required for
pulmonary surfactant to function but does confer immune effects to the organism.