Respiratory syncytial virus, or RSV, causes disease through a cascade that begins with a surprisingly selective invasion of the airway and unfolds into a battle between viral replication and the immune system’s attempt to contain it. RSV is an enveloped RNA virus that targets ciliated cells lining the airways, and much of the damage it causes stems not from the virus killing those cells outright but from the immune response it provokes and the structural changes it imposes on the airway lining. Understanding how the virus is built, how it enters cells, and how the immune system responds reveals why certain people, especially infants and older adults, are so vulnerable.
The Anatomy of RSV
RSV belongs to the family Paramyxoviridae and is classified as an enveloped, nonsegmented negative-strand RNA virus. It is the most complex member of its family in terms of gene and protein count, encoding eleven proteins from ten genes.1PubMed Central. Respiratory syncytial virus: virology, reverse genetics, and pathogenesis of disease The virus particle is wrapped in a lipid membrane studded with three surface proteins, but two of them do most of the heavy lifting during infection: the attachment glycoprotein (G) and the fusion glycoprotein (F).
The G protein is the virus’s grappling hook. It latches onto receptors on the surface of airway cells, tethering the virus in place. A conserved region of G contains a motif that mimics a human immune-signaling molecule called CX3CL1, allowing it to bind the CX3CR1 receptor found on both airway cells and certain immune cells.2PubMed Central. Respiratory syncytial virus G protein CX3C motif impairs human airway epithelial and immune cell responses This molecular mimicry does double duty: it helps the virus attach to cells and simultaneously interferes with immune signaling, a theme that comes up repeatedly in RSV pathophysiology.
The F protein handles the next step. It exists in a spring-loaded “prefusion” shape on the virus surface. When triggered, it snaps into a dramatically different “postfusion” shape, and that mechanical rearrangement pulls the viral membrane and the cell membrane together until they merge.3PubMed. Respiratory syncytial virus entry mechanism in host cells: A general overview X-ray crystallography has shown that the prefusion and postfusion shapes of F are so different that antibodies raised against one form may not recognize the other well, a discovery that reshaped vaccine design.4PubMed Central. Structural basis for immunization with postfusion respiratory syncytial virus fusion F glycoprotein (RSV F) to elicit high neutralizing antibody titers Structural studies have also revealed that the prefusion F trimer is not rigid; its apex can tilt by roughly 10 to 20 degrees between two distinct states, transiently exposing a central cavity that matters for both viral function and drug design.5Nature Communications. Transient opening of trimeric prefusion RSV F proteins
How RSV Gets Into Airway Cells
RSV does not infect the airway indiscriminately. It overwhelmingly targets ciliated epithelial cells, the hair-like cells whose rhythmic beating moves mucus up and out of the lungs. CX3CR1, the receptor for the G protein, is concentrated on the apical surface of these ciliated cells and even on the cilia themselves. When researchers blocked CX3CR1 with an antibody in human airway cultures, RSV infection dropped significantly, and mice lacking CX3CR1 were less susceptible to infection.6PLOS Pathogens. Respiratory Syncytial Virus Uses CX3CR1 as a Receptor on Primary Human Airway Epithelial Cultures
CX3CR1 is not the only way in. The G protein can also bind heparan sulfate proteoglycans on cell surfaces, providing a secondary attachment point. Meanwhile, the F protein interacts with its own set of receptors, including nucleolin, insulin-like growth factor 1 receptor (IGF1R), and intercellular adhesion molecule 1 (ICAM-1).7Frontiers in Cellular and Infection Microbiology. Receptors for Respiratory Syncytial Virus Infection and Host Factors Regulating the Life Cycle of Respiratory Syncytial Virus This redundancy matters: even if one receptor is blocked or absent, the virus has alternate routes to get inside. Nucleolin, for instance, is a protein normally found inside cells that gets shuttled to the surface, where RSV can exploit it.8PubMed Central. Targeting Host Cell Surface Nucleolin for RSV Therapy: Challenges and Opportunities
Once the virus is tethered, the F protein fires. Its transition from the prefusion to postfusion conformation drives the actual membrane fusion that lets the viral contents spill into the cell. Small-molecule drugs have been developed that wedge into the central cavity of prefusion F, physically preventing it from rearranging and thus blocking entry.9PubMed Central. Molecular mechanism of respiratory syncytial virus fusion inhibitors This illustrates why the structural flexibility of prefusion F is therapeutically interesting: that cavity is a druggable target. Another entry inhibitor, CL-A3-7, works differently by blocking the interaction between the F protein and IGF1R on host cells.10PubMed Central. A new mechanism of respiratory syncytial virus entry inhibition by small-molecule to overcome K394R-associated resistance
Replication Inside the Cell
After fusion, the viral RNA and its associated proteins are released into the cell’s cytoplasm. RSV carries a negative-sense RNA genome, which means it cannot be read directly by the cell’s protein-making machinery. The virus must first copy its genome into a readable positive-sense strand using its own RNA-dependent RNA polymerase, a large protein called L that works in partnership with a smaller cofactor called P. Cryo-electron microscopy has revealed that L and P form a striking complex in which four copies of P wrap around L in distinct conformations, creating a tentacle-like arrangement that positions the polymerase for both transcription and genome copying.11Cell. Structure of the RSV L-P Complex Determines the Mechanism of RSV Transcription and Replication The polymerase’s structure also includes a capping domain that is distinct from those found in host cells, making it a potential target for antiviral drugs.12Nature Communications. Cryo-EM structure of the respiratory syncytial virus RNA polymerase
As new viral proteins accumulate, F protein expressed on the surface of an infected cell can fuse that cell with its neighbors, creating giant multinucleated blobs called syncytia. This is literally how the virus got its name: “syncytial” refers to these fused cell masses. Syncytia allow the virus to spread from cell to cell without ever being exposed to antibodies in the surrounding fluid, a built-in immune evasion trick. A host receptor called RAGE can interfere with F-protein-mediated syncytium formation and may help protect the lower airways from this kind of damage.13PubMed Central. RAGE inhibits human respiratory syncytial virus syncytium formation by interfering with F-protein function
Damage to the Airway
RSV’s preference for ciliated cells sets the stage for significant airway damage. In laboratory studies of human bronchial tissue, RSV infection causes ciliary activity to slow within thirty minutes. By two hours, no cilia are still beating. Over three to five days, cilia are completely lost from the cell surface, while many cells fuse into syncytia, round up, or slough off entirely.14JAMA Otolaryngology–Head & Neck Surgery. Respiratory Syncytial Virus and Human Bronchial Epithelium This destruction of the mucociliary escalator, the system that sweeps debris and pathogens out of the lungs, is a major reason RSV leads to airway obstruction in small children, whose airways are already narrow.
An intriguing nuance is that in more physiologically realistic airway culture models, RSV infection can persist for months without obvious cell death, suggesting the virus itself is not inherently cytotoxic. The severe damage seen in actual patients is largely driven by the immune response rather than by the virus tearing cells apart on its own.15PubMed Central. Respiratory syncytial virus infection of human airway epithelial cells is polarized, specific to ciliated cells, and without obvious cytopathology More recent work has shown that at higher viral loads, RSV triggers a dose-dependent shutdown of genes responsible for building cilia, presenting antigens to the immune system, and activating innate sensing pathways, including key interferon and pattern-recognition pathways.16PubMed Central. Respiratory syncytial viral load drives ciliated cell dedifferentiation and suppresses antiviral immunity
RSV infection also ramps up mucus production. The virus upregulates the expression of MUC5AC, a major mucus gel-forming protein, in airway epithelial cells.17PubMed Central. ITGB4 deficiency induces mucus hypersecretion by upregulating MUC5AC in RSV-infected airway epithelial cells Excessive mucus, combined with the loss of ciliary clearance and inflammatory cell infiltration, plugs the small airways of infants and produces the wheezing and breathing difficulty that characterize RSV bronchiolitis.
How the Innate Immune System Detects RSV
The body’s first-line defense against RSV relies on pattern recognition receptors that detect viral components. These include Toll-like receptors, RIG-I-like receptors, and NOD-like receptors, each sensing different molecular signatures of the virus.18BMB Reports. Innate immune recognition of respiratory syncytial virus infection Once activated, these sensors trigger the production of interferons, the body’s primary antiviral alarm signals, along with inflammatory cytokines and chemokines that recruit immune cells to the site of infection.19PubMed. Pattern recognition receptors for respiratory syncytial virus infection and design of vaccines
Type I interferons are particularly important in determining how RSV infection plays out. A strong early interferon response is generally associated with milder disease, faster viral clearance, and less lung damage. In human infants, however, the type I interferon response to RSV tends to be weak. Two RSV nonstructural proteins, NS1 and NS2, actively suppress type I interferon production, and the immature immune systems of young infants are already less equipped to generate a robust interferon burst.20Frontiers in Immunology. Role of Type I Interferon (IFN) in the Respiratory Syncytial Virus (RSV) Immune Response and Disease Severity Meanwhile, RSV strongly induces type III interferons in the lungs, and these have been associated with more severe disease in children. A study of young children found that higher mucosal concentrations of type III interferon (IFN-λ2/3) were linked to milder illness and reduced odds of hospitalization, while elevated levels of the inflammatory cytokine IL-6 increased hospitalization risk in older infants.21PubMed Central. Type III Interferons, Viral Loads, Age, and Disease Severity in Young Children With Respiratory Syncytial Virus Infection
How RSV Evades the Immune System
RSV is not a passive target. Its NS1 and NS2 proteins are produced early and in large quantities during infection, and they function as dedicated saboteurs of the innate immune response.22PubMed Central. Respiratory Syncytial Virus’s Non-structural Proteins: Masters of Interference NS2 directly grabs the inactive forms of the RIG-I and MDA5 sensors and prevents them from being activated through ubiquitination, a chemical tagging process that is necessary for downstream interferon signaling. Structural studies have pinpointed the N-terminal region of NS2 as the critical docking site for this interaction; mutations there sharply reduce RIG-I binding and let interferon production recover.23PubMed Central. Structural basis for IFN antagonism by human respiratory syncytial virus nonstructural protein 2
The suppression extends beyond epithelial cells. NS1 in particular suppresses the maturation of dendritic cells, the sentinels that bridge innate and adaptive immunity. Dendritic cells infected with RSV lacking the NS1 gene show increased expression of surface maturation markers and produce more cytokines and chemokines. This effect is largely due to NS1’s suppression of type I interferon signaling, because blocking the interferon receptor on dendritic cells largely undoes the maturation boost seen with NS1-deleted virus.24PubMed Central. Nonstructural proteins 1 and 2 of respiratory syncytial virus suppress maturation of human dendritic cells The result is a weaker, less well-orchestrated adaptive immune response, which helps explain why RSV reinfection is common throughout life despite the virus being antigenically relatively stable.
The G protein adds another layer of immune interference. Its CX3C motif does not activate CX3CR1 the way the natural ligand CX3CL1 does, but it effectively blocks CX3CL1 from activating the receptor in a dose-dependent manner. This competitive inhibition disrupts the normal trafficking and function of immune cells that rely on CX3CR1 signaling, including certain monocytes and T cells.25npj Viruses. Respiratory syncytial virus glycoprotein G impedes CX3CR1-activation by CX3CL1 and monocyte function
The Adaptive Immune Response and Antibody Targets
When the adaptive immune system eventually engages, both T cells and antibodies play important roles in clearing the virus, though the relationship is complicated. In mouse models, both CD4 and CD8 T cells contribute to viral clearance but also drive disease symptoms.26PubMed Central. Determining the breadth of the respiratory syncytial virus-specific T cell response Regulatory T cells help keep CD8 T cells from causing excessive lung damage; depleting them in mice leads to delayed viral clearance, greater weight loss, and worse airway restriction. In mice without regulatory T cells, the remaining CD8 T cells become more aggressive, producing both IFN-γ and TNF-α simultaneously, which likely drives the increased tissue damage.27The Journal of Immunology. Foxp3+ CD4 Regulatory T Cells Limit Pulmonary Immunopathology by Modulating the CD8 T Cell Response during Respiratory Syncytial Virus Infection
On the antibody side, both the F and G proteins are targets for neutralizing antibodies, but F is the more potent target. Classic experiments using monoclonal antibodies showed that only antibodies against the F protein achieved complete neutralization, while antibodies against either F or G could achieve partial neutralization. Mixing antibodies against both proteins enhanced the overall neutralizing effect.28PubMed Central. Neutralization of respiratory syncytial virus by individual and mixtures of F and G protein monoclonal antibodies This is why most modern vaccines and monoclonal antibodies center on the prefusion form of F, which presents the most potent neutralizing epitopes. One key site, called antigenic site Ø, sits at the apex of the prefusion F trimer and is the target of some of the most potent neutralizing antibodies discovered to date.29PubMed Central. Discovery of a Prefusion Respiratory Syncytial Virus F-Specific Monoclonal Antibody That Provides Greater In Vivo Protection than the Murine Precursor of Palivizumab
Antibodies against the G protein’s central conserved region are also valuable. Crystal structures of human antibodies bound to this region show they recognize a folded structure called the cystine noose, which is conserved across RSV subtypes.30PLOS Pathogens. Structural basis for recognition of the central conserved region of RSV G by neutralizing human antibodies Vaccine strategies that combine prefusion F with G protein fragments have shown they can redirect antibody responses toward the CX3C motif and the central conserved region of G, sites where antibodies could block early viral attachment.31npj Vaccines. RSV pre-fusion F protein enhances the G protein antibody and anti-infectious responses
Why Infants and Older Adults Are Most Vulnerable
RSV infects virtually everyone by age two, and reinfection occurs throughout life, but severe disease clusters at the extremes of age. In infants, the immune system is simply not yet ready to mount a coordinated defense. Cellular immunity is immature, and virus-specific antibody levels are low, particularly in infants born before maternal antibodies have had time to transfer fully across the placenta.32PubMed. Immunological mechanisms of severe respiratory syncytial virus bronchiolitis The narrow airways of young infants compound the problem: even modest swelling, mucus accumulation, and cell debris can obstruct airflow in a way that would be easily tolerated in a larger airway.
At the other end of the age spectrum, older adults face a different but overlapping set of vulnerabilities. Age-related declines in both mechanical airway defenses and antiviral immunity leave them more susceptible, while chronic diseases like heart failure and COPD amplify the risk.33PubMed Central. High seroprevalence and high risk: why are older adults more prone to respiratory syncytial virus? Despite having been exposed to RSV many times over their lives and carrying high levels of circulating antibodies, older adults still develop severe infections because their immune parameters decline across the board: impaired viral clearance and distorted cytokine profiles contribute to worse outcomes compared to younger adults.34PubMed. Review of impaired immune parameters in RSV infections in the elderly
RSV and the Long Shadow of Childhood Wheezing
One of the most clinically consequential aspects of RSV pathophysiology is its association with later respiratory problems. Children who experience RSV-related lower respiratory tract infections in early life have roughly double the risk of developing asthma compared to children who did not have such infections, based on a meta-analysis that pooled multiple studies.35PubMed Central. Association between early-life respiratory syncytial virus infection and the risk of asthma: a meta-analysis Whether RSV itself causes asthma or simply marks children whose airways were already predisposed remains debated. The same meta-analysis found no significant difference in asthma risk between children who had RSV-specific lower respiratory infections and those who had non-RSV respiratory infections, suggesting the inflammation of severe early infection, regardless of which virus causes it, may be the key driver.36Annals of Allergy, Asthma & Immunology. The impact of respiratory syncytial virus on asthma development and exacerbation
The biological plausibility is strong, though. RSV’s ability to drive mucus overproduction through MUC5AC upregulation, destroy ciliated cells, and reshape the local immune environment could plausibly alter airway development during a critical window in infancy.37PubMed Central. Respiratory syncytial virus infection‐induced mucus secretion by down‐regulation of miR‐34b/c‐5p expression in airway epithelial cells
How RSV Evolves and Stays Ahead
RSV exists as two major groups, A and B, distinguished primarily by differences in the G glycoprotein. The G gene is by far the most variable part of the RSV genome, accumulating more mutations than any other gene at both the nucleotide and amino acid levels.38Genome Biology and Evolution. Genome-Wide Analyses of Human Respiratory Syncytial Viruses Provide Insights into Evolutionary Dynamics This makes sense given that G is a major target of the immune response: the virus faces constant selective pressure to change its surface. Genome-wide analyses from 1977 to 2019 estimated that RSV-A evolves at roughly 1.5 × 10⁻³ nucleotide substitutions per site per year and RSV-B at about 1.9 × 10⁻³, with several sites in the G protein under strong positive selection.39PubMed Central. Genetic diversity and molecular evolution of human respiratory syncytial virus A and B No intergroup recombination has been observed, meaning the two subtypes evolve independently.
The F protein, by contrast, is far more conserved across RSV strains and subtypes. This is one reason why F-based vaccines and monoclonal antibodies provide broad protection: the virus cannot easily mutate its fusion machinery without losing function. Newer antibodies are being designed to target especially conserved patches within F, such as site V, to further reduce the risk of escape mutants.40Nature Communications. A potent broad-spectrum neutralizing antibody targeting a conserved region of the prefusion RSV F protein
From Pathophysiology to Prevention
Nearly every step in RSV’s lifecycle described above is now being targeted therapeutically. Fusion inhibitors that lock prefusion F in place, entry blockers that interrupt the F-IGF1R interaction, and monoclonal antibodies against antigenic sites on both F and G are all either approved or in development. The two approved monoclonal antibodies for infant protection, palivizumab and the longer-acting nirsevimab, both target prefusion F. Maternal vaccination takes advantage of the same biology from a different angle: vaccinating pregnant women with prefusion-F-based immunogens boosts the mother’s memory B cells and enhances the transfer of neutralizing antibodies across the placenta, protecting newborns during their first vulnerable months.41PubMed Central. Maternal Respiratory Syncytial Virus (RSV) Vaccination: Current Status and Comparison to Monoclonal Antibodies (mAbs) for RSV Prevention in Infants and Children In clinical trials, one licensed maternal RSV vaccine showed efficacy against severe medically attended RSV lower respiratory infections of about 82% within the first 90 days of the infant’s life.42PubMed Central. Maternal RSV immunization: clinical efficacy, immunological mechanisms and public health implications for preventing infant lower respiratory tract infection
A recent and somewhat unsettling finding is that sub-neutralizing levels of anti-F antibodies can, under certain laboratory conditions, enhance RSV infection of cells that carry Fc receptors. In experiments using a human monocyte cell line, both palivizumab and nirsevimab at very low dilutions promoted rather than blocked RSV entry, an effect that disappeared when the Fc portion of the antibody was removed.43Frontiers in Immunology. Sub-neutralizing levels of antibodies against RSV F protein enhance RSV infection via Fc-FcγR interactions Whether this antibody-dependent enhancement occurs in actual human infections is unclear, but the finding underscores a recurring lesson in RSV biology: even the immune system’s tools can cut both ways when they fall below the threshold needed to fully neutralize the virus. It is a reminder that the same structural and immunological details that make RSV so difficult to fight also guide the increasingly precise strategies being developed to prevent it.