Human surfactant protein A is a water soluble, collagen like domain containing, innate immune system collectin. Surfactant protein A is similar to SP-D. Surfactant protein A is a major protein of pulmonary surfactant. This is the material secreted to reduce the surface tension at the alveolar air-liquid interfaces. Surfactant protein A is used for bacterial opsonisation in the alveoli and is a part of innate immune system. Surfactant protein is used for marking the bacterial cells for phagocytosis by the macrophages in the alveoli. Negative feedback limiting the secretion of pulmonary surfactant is one of the major role of surfactant protein a. it is not used to function but to confer immune effects in the organism.

Pulmonary surfactants are lipoproteins which are synthesized and secreted by alveolar type II celss. These surfactant proteins are made up of phospholipids, proteins and cholesterol. Surfactant protein A is 34-36 kDa which regulates the surface tension and also involved in innate responses in the lungs. They are able to bind to different pathogens which include bacteria, fungi, viruses and allergens like mites and pollen grains. Surfactant protein A is present in most abundance and in humans it is encoded by the genes SP-A1 and SP-A2. Multiple m-RNA transcripts are produced by SP-A gene. The native SP-A protein forms a flower bouquet structure made up of six SP-A trimers. Surfactant protein A is a member of collectin family and found together on chromosome 10 in humans. Inflammatory mediators like TNF are regulated by surfactant protein A. various changes in the levels of SP-A in lung or serum are associated with progression of idiopathic pulmonary fibrosis. Various pulmonary diseases like acute respiratory distress syndrome show a decrease in the levels of surfactant protein A while pulmonary alveolar protienosis show increase levels of surfactant protein A in lungs.