Surfactant Protein B (SP-B) is a protein found within the lungs, playing a role in breathing. It is one of several proteins that work with lipids to form pulmonary surfactant, a substance lining the air sacs of the lungs. This surfactant helps maintain the function of these air sacs, which are responsible for gas exchange. Understanding SP-B’s role provides insight into how lungs operate and what happens when this system is disrupted.
Understanding Lung Surfactant and SP-B
Lung surfactant is a mixture of phospholipids and proteins that coats the inner surface of the alveoli, the air sacs in the lungs where oxygen enters the bloodstream and carbon dioxide is released. This substance is approximately 90% lipids, primarily phosphatidylcholine, and 10% proteins. The protein component includes Surfactant Protein A (SP-A), SP-B, SP-C, and SP-D, each contributing to surfactant function.
Surfactant Protein B is a hydrophobic protein, meaning it repels water, allowing it to interact with surfactant’s lipid components. This protein is synthesized and secreted by Type II alveolar cells (Type II pneumocytes) in the lungs. These cells produce and package lipids and proteins into structures called lamellar bodies before release into the alveolar lining fluid.
SP-B’s Role in Breathing
The function of SP-B is to facilitate the spread and organization of surfactant’s lipid components across the alveolar surface. Without SP-B, the lipids cannot arrange themselves into a monolayer at the air-liquid interface. This organization is necessary for the surfactant to reduce surface tension.
SP-B lowers the surface tension within the alveoli, preventing them from collapsing when a person exhales. This reduction in surface tension increases lung compliance, making it easier for the lungs to inflate with each breath. If SP-B is absent or dysfunctional, surface tension remains high, making it difficult for the alveoli to stay open, which impairs gas exchange and increases the effort required for breathing.
When SP-B Levels Are Low or Absent
When the body produces insufficient or abnormal SP-B, respiratory complications can arise. The primary condition linked to this deficiency is congenital SP-B deficiency, also known as Hereditary Surfactant Protein B Deficiency. This rare inherited disorder is caused by mutations in the SFTPB gene, presenting as an autosomal recessive condition, meaning a child inherits an abnormal gene copy from each parent.
Infants with complete SP-B deficiency develop respiratory distress syndrome (RDS) shortly after birth. Symptoms include rapid breathing (tachypnea), grunting sounds, nasal flaring, and a grayish-blue discoloration of the skin (cyanosis) due to low oxygen levels. This condition can progress to fatal respiratory failure within months if not properly managed, as the lungs struggle to maintain inflation and gas exchange.
Diagnosis and Management of SP-B Deficiency
Diagnosing SP-B deficiency involves clinical assessment and laboratory tests. Healthcare providers may suspect the condition in newborns with unexplained respiratory failure that does not resolve within several days. Genetic testing for mutations in the SFTPB gene is the definitive method for confirming the diagnosis, taking several weeks for results. Analysis of bronchoalveolar lavage fluid, which involves collecting fluid from the lungs, can also indicate low levels of SP-B.
Management strategies for SP-B deficiency are supportive. Surfactant replacement therapy, which involves administering exogenous surfactant directly into the lungs, may provide transient improvement in respiratory status. Mechanical ventilation is required to support breathing and maintain oxygenation. For infants with severe congenital SP-B deficiency, lung transplantation is the only option for long-term survival, though the pre-transplant period carries a high risk.