What Is Surfactant Protein B? Definition and What Research Reports

Definition

Surfactant protein B (SP-B) is a small, hydrophobic, amphipathic protein found in pulmonary surfactant — the lipid–protein film that lines the alveoli of the lung. It is encoded by the SFTPB gene and synthesised by alveolar type II epithelial cells as a larger precursor (proSP-B) that is proteolytically processed into the mature protein and packaged into lamellar bodies before secretion. Functionally, SP-B is described in the literature as the surfactant component most closely tied to the formation and stability of the surface-active film at the air–liquid interface, which is why loss-of-function variants in the gene are associated with severe neonatal respiratory disease. The term is used in two distinct ways: to describe the native human protein (and its precursor forms) and, in peptide chemistry and drug-delivery work, to describe short synthetic peptides derived from or modelled on SP-B sequences.

This page is for educational purposes only and is not medical advice; consult a licensed physician for any health-related question. Nothing here describes how any substance should be used, and SP-B-derived peptides discussed below appear in the literature as laboratory and preclinical research tools rather than as consumer products.

What Class of Molecule Is It, and Where Does It Come From?

SP-B belongs to the saposin-like protein family and is classed as a hydrophobic surfactant protein, distinguishing it from the hydrophilic collectins SP-A and SP-D, which are more closely associated with innate immune recognition. Its biological source is the alveolar type II cell; in animal-derived therapeutic surfactant preparations, SP-B is present because those preparations are lipid extracts of lung tissue. Because the mature protein is intensely hydrophobic and difficult to produce recombinantly in a correctly folded, active state, much of the peptide-science literature works instead with SP-B peptide mimics: short helical sequences intended to reproduce the lipid-interacting behaviour of the parent protein.

How the Term Is Used in Peptide Research

Outside respiratory physiology, SP-B appears most often in peptide research as a membrane-active sequence. Researchers reported that surfactant protein B promoted cytosolic siRNA delivery by adopting a virus-like mechanism of action in a 2021 nanotechnology study (PMID 33724778). A 2025 controlled-release paper extended that line of work by examining surfactant protein B-derived peptides as endosomal escape enhancers for pulmonary delivery of siRNA (PMID 40010411). In both cases the peptide is of interest because it interacts with lipid bilayers — the same property that makes native SP-B important for surfactant film behaviour.

A separate strand of peptide work has used SP-B mimics as probes of protein–receptor interactions. A 2023 molecular-sciences study reported that lung surfactant protein B peptide mimics interacted with the human ACE2 receptor (PMID 37446012). Findings of that kind are interaction-level observations in experimental systems and do not, in the cited literature, describe a clinical application.

What the Published Literature Reports

SP-B deficiency

The clearest disease association is genetic. A 2025 cohort report described surfactant protein B deficiency in the RespiRare cohort, characterising the clinical course of affected children (PMID 39632082). A 2023 case report likewise documented congenital surfactant protein B (SP-B) deficiency in an individual patient (PMID 37455866). Together these publications illustrate why SP-B is treated as non-redundant: unlike some surfactant components, its absence is reported in the literature as producing severe neonatal respiratory failure rather than a subtle phenotype.

SP-B as a measured marker

Several groups have measured SP-B in blood or airway samples and compared groups. A 2023 study assessed surfactant protein B plasma levels and their reliability as a biomarker in COPD patients (PMID 36672632). In neonatal medicine, researchers reported that surfactant protein B and A concentrations were increased in neonatal pneumonia (PMID 26107393). In asthma, a 2022 allergy paper reported down-regulated surfactant protein B in obese asthmatics (PMID 35294785). The direction of change therefore differs by condition and by the sample type measured, which is one reason the biomarker literature remains exploratory rather than settled.

SP-B has also been discussed in the context of infection biology more broadly: a 2009 review examined pulmonary surfactant and tuberculosis, situating surfactant components within host–pathogen interactions at the alveolar surface (PMID 20006297).

Circulating SP-B and lipoprotein studies

A less obvious research thread concerns SP-B measured in serum alongside lipoprotein function. A 2019 cardiology study reported that immature surfactant protein-B impaired the antioxidant capacity of HDL (PMID 30857841). An earlier 2017 paper examined sports and HDL quality as reflected by serum amyloid A and surfactant protein B (PMID 29104456). These studies treat SP-B — particularly incompletely processed forms — as an HDL-associated analyte rather than as a lung-function measure.

Immune and Adverse-Event Findings: What Studies Report

Because SP-B is a tissue-restricted protein, it has been examined as a potential autoantigen. A 2024 respiratory-medicine study reported that autoimmunity against surfactant protein B was associated with pneumonitis during checkpoint blockade (PMID 38626354). That finding concerns anti-SP-B immune responses arising in patients receiving cancer immunotherapy; the study describes an association observed in that clinical setting and does not report on administration of SP-B or SP-B peptides to people. Within the verified literature summarised on this page, no human safety or tolerability data exist for SP-B-derived research peptides, and the delivery-focused papers cited above (PMID 33724778, PMID 40010411) are laboratory and preclinical in scope.

Reading the SP-B Literature Carefully

Three distinctions help when interpreting papers that use this term:

• Protein form matters. Mature SP-B, proSP-B and "immature" SP-B are not interchangeable analytes; the HDL-related work specifically examined immature surfactant protein-B (PMID 30857841).
• Compartment matters. Airway or lavage measurements and plasma measurements answer different questions, as illustrated by the plasma-biomarker evaluation in COPD (PMID 36672632).
• Protein versus peptide mimic. Results obtained with short synthetic sequences, such as the ACE2-interaction work (PMID 37446012), describe the peptide tested and not necessarily the full-length native protein.

In summary, the published record treats SP-B simultaneously as an essential structural component of surfactant whose genetic loss is reported to cause severe neonatal disease (PMID 39632082), as an exploratory biomarker whose levels have been reported to shift in different directions across respiratory conditions (PMID 35294785), and as a source of membrane-active peptide sequences used in laboratory delivery research (PMID 40010411). This entry is definitional and summarises what the study literature reports; it is not medical advice and contains no guidance on use.

References

• Autoimmunity Against Surfactant Protein B Is Associated with Pneumonitis During Checkpoint Blockade (American Journal of Respiratory and Critical Care Medicine, 2024)
• Down-regulated surfactant protein B in obese asthmatics (Clinical and Experimental Allergy, 2022)
• Pulmonary surfactant and tuberculosis (Tuberculosis, 2009)
• Surfactant protein B-derived peptides as endosomal escape enhancers for pulmonary delivery of siRNA (Journal of Controlled Release, 2025)
• Surfactant Protein B Plasma Levels: Reliability as a Biomarker in COPD Patients (Biomedicines, 2023)
• Lung Surfactant Protein B Peptide Mimics Interact with the Human ACE2 Receptor (International Journal of Molecular Sciences, 2023)
• Immature surfactant protein-B impairs the antioxidant capacity of HDL (International Journal of Cardiology, 2019)
• Surfactant protein B and A concentrations are increased in neonatal pneumonia (Pediatric Research, 2015)
• Surfactant protein B deficiency: the RespiRare cohort (Thorax, 2025)
• Congenital surfactant protein B (SP-B) deficiency: a case report (Pan African Medical Journal, 2023)
• Sports and HDL-Quality Reflected By Serum Amyloid A and Surfactant Protein B (International Journal of Medical Sciences, 2017)
• Surfactant Protein B Promotes Cytosolic SiRNA Delivery by Adopting a Virus-like Mechanism of Action (ACS Nano, 2021)