The proteins make roughly 10 percent of the mass and improve surfactant adsorption to the saline-air interface and cells in the alveolus, including type II aveolar cells and macrophages. Their function is to improve flexibility of the layer and they contribute to lowering its surface energy on compression. SP-A binds phospholipids, but experimental mice can survive without it. SP-A seems to inhibit surfactant production and can bind to receptors on the type II cells. SP-D is poorly bound to phospholipids and seems to enhance phagocytosis of bacteria.
Immunologists call it a collectin, rather than an opsonin.
Recent theory suggests a role for collectins in clearing all the inhaled pollution without generating any feature of inflammation in the normal lung. The immunological role of type II alveolar cells is supported by the observation that they can make proteins that cause neutrophil chemotaxis. Surfactant has a high rate of turnover and is replaced with a half life of about 10 hours. The discussion of Pulmonary Surfactant is readily made complicated, because much remains to be discovered. The component proteins are adsorbed at different rates onto the macrophages.