SP-B is a critical protein for lung function, and is found in the context of pulmonary surfactant.  Understanding surfactant is important to gaining a full understanding of SP-B.  Surfactant is a mixture of lipids and proteins that coats the inside of alveoli and is essential for life due to its key role in preventing alveolar collapse at low lung volumes.  In the absence of surfactant, the surface tension at the gas/fluid interface prevents inhalation at standard pressure, but surfactant minimizes surface tension to values near zero and allows for normal breathing. It is also known to have a role in both the immune response and inflammation control.
Surfactant deficiency is a common cause of respiratory disease.  Respiratory distress syndrome (RDS) is a particularly well-known instance of surfactant deficiency because it has a high mortality rate among preterm babies, a variety of other conditions are related to surfactant levels and composition.
Surfactant is composed of primarily lipids (90% by weight), and proteins make up only the remaining 10%. The following two sections will address the lipid and protein components respectively.