RSV: Structure, Immune Response, and Diagnostic Techniques

Respiratory syncytial virus, better known as RSV, is an enveloped virus carrying a single strand of RNA that infects nearly every child on Earth before the age of two and remains a lifelong threat to older adults and anyone with a weakened immune system. Its surface is studded with proteins that hijack airway cells, and it deploys a surprisingly clever set of tools to dodge the immune response once inside.¹ Understanding how the virus is built, how the body fights it, and how clinicians actually detect it sheds light on why RSV has been so difficult to control and why the recent wave of vaccines and antibody treatments was decades in the making.

What RSV Looks Like Up Close

RSV belongs to the family Paramyxoviridae and carries its genetic information as a single, continuous strand of negative-sense RNA. “Negative-sense” simply means the viral RNA cannot be read directly by the cell’s protein-making machinery; the virus must first copy it into a mirror-image strand before any viral proteins can be produced. The genome encodes ten genes that give rise to eleven proteins, nine of which were originally mapped through cloned complementary DNA sequences in the early 1980s.²

The two proteins most relevant to infection sit on the viral surface. The G glycoprotein handles attachment: it latches onto airway cells using a motif that mimics a human immune-signaling molecule called fractalkine, essentially tricking receptors on the cell surface into letting the virus dock.³ The F (fusion) glycoprotein then does the heavy lifting of entry. It exists initially in a spring-loaded “prefusion” shape. Once triggered, it snaps into a dramatically different “postfusion” form, and this shape change physically drives the viral envelope and the host cell membrane together, merging them so the viral contents can spill inside.⁴ The crystal structure of the postfusion F trimer, solved at 2.8-angstrom resolution, showed that certain sites targeted by neutralizing antibodies survive this dramatic rearrangement, a finding that has shaped vaccine design ever since.

How RSV Enters and Commandeers Host Cells

The entry process starts when the G protein binds a receptor on the surface of an airway epithelial cell, selecting which cell types are susceptible. That initial contact positions the F protein close to the host membrane. The prefusion F protein is metastable, meaning it sits in a high-energy state waiting for the right cue. When rearrangement begins, a previously buried fusion peptide swings outward, inserts into the host membrane, and the protein refolds around it, pulling the two membranes together until they merge.⁵ The virus does not need to be swallowed whole by the cell through an endosome, the way some other viruses enter. Instead, RSV can fuse directly at the cell surface, which has implications for how quickly it begins replicating once contact is made.

After the viral genome is released into the cell’s interior, RSV’s own polymerase complex transcribes the negative-sense RNA into messenger RNA, which the cell’s ribosomes then translate into viral proteins. New viral particles assemble at the cell surface in distinctive filamentous shapes that protrude outward. Interestingly, this filament formation does not depend on the cell’s own structural skeleton. Disrupting actin or tubulin, the two major components of the internal scaffolding that cells use to maintain shape and move cargo around, did not stop RSV from building its filamentous particles.⁶ The virus appears to deform the host membrane and elongate its particles through its own protein-driven mechanism, though depolymerized actin clumps sometimes served as anchoring points.

The G Protein’s Double Life

The G glycoprotein does more than simply attach to cells. It comes in two forms: one anchored in the viral membrane and one that is actively secreted into the surrounding fluid. This secreted or “soluble” G protein (sG) functions as a decoy and an immune disruptor. In laboratory experiments, wild-type RSV that produced sG was less susceptible to neutralization by G-specific and broadly RSV-specific antibodies than a modified virus engineered to make only the membrane-bound form. The difference disappeared when soluble G was physically removed from virus preparations, confirming that sG was acting as a decoy that soaked up antibodies before they could reach the actual virus particle.⁷

In mice, the evasion effect turned out to be even broader. The virus that produced sG was also less sensitive to F-specific antibodies, not just G-specific ones. That added protection required inflammatory cells in the lung and depended on a receptor on immune cells that recognizes the tail end of antibodies. In other words, sG was not just absorbing antibodies; it was also interfering with the way immune cells use antibodies to flag and destroy infected cells. Beyond antibody decoy work, sG binds a pattern-recognition receptor called TLR2 on uninfected neighboring cells, triggering inflammatory signaling that primes those cells for a form of inflammatory cell death called pyroptosis. When RSV subsequently infects those primed cells, they rupture more readily, releasing new virus particles into the airway.³ The virus effectively pre-conditions the surrounding tissue to help it spread.

How RSV Shuts Down Interferon Defenses

Cells under viral attack normally produce interferons, signaling molecules that warn neighboring cells to ramp up their antiviral defenses. RSV carries two nonstructural proteins, NS1 and NS2, whose primary job is to suppress this alarm system. NS1 and NS2 cause a sharp drop in the levels of a key signaling molecule called STAT2, which is needed for cells to respond to interferon once it arrives. Without STAT2, the interferon signal goes unanswered even though it may have been sent.⁸ When researchers deleted NS1 and NS2 from the virus, STAT2 levels stayed normal and cells retained their ability to respond to interferon.

NS2 also attacks an earlier step in the chain. It physically binds to inactive forms of two intracellular sensors, RIG-I and MDA5, that cells use to detect foreign RNA. By blocking a chemical modification those sensors need to stay switched on, NS2 prevents them from sustaining the downstream signaling that would ordinarily amplify the interferon response.⁹ RSV therefore attacks the interferon pathway at multiple points simultaneously, both at the level of detection and at the level of response. This multi-pronged sabotage helps explain why RSV reinfects people throughout life: the initial immune response is partially blunted every time, leaving the body with an incomplete memory of the encounter.

The Body’s Response and Its Paradoxes

Despite RSV’s evasion tactics, the immune system does mount a vigorous response. Neutrophils, the first-responder white blood cells that flood into infected tissue, arrive in large numbers early in RSV infection. Their recruitment is driven heavily by a chemokine called IL-8 in humans, and the intensity of this neutrophil surge is positively linked to how sick the patient becomes.¹⁰ In mouse models, the signaling pathway that recruits neutrophils into the lungs depends on MyD88 and TRIF, two adapter molecules used by innate immune receptors. Mice lacking both of these molecules failed to produce the neutrophil-attracting chemokine CXCL1 in any lung cell population during RSV infection, and neutrophil recruitment collapsed.¹¹

T cells clear the virus, but they come with a cost. Depleting CD8+ T cells in mice reduced weight loss and lung damage during RSV infection, even though it delayed viral clearance. CD4+ T cell depletion had a smaller but similar effect. RSV-specific T cells are therefore both essential for getting rid of the virus and a direct cause of tissue damage in the process.¹² This dual role helps explain why the youngest and oldest patients fare worst: infants have immature T cell responses biased toward an unhelpful pattern, while older adults may have accumulated years of imperfect immune memory that triggers inflammation without efficiently clearing the virus.

Why Infants Are Especially Vulnerable

Newborns inherit RSV-specific antibodies from their mothers across the placenta, but these maternal antibodies fade quickly. Data from a birth cohort in Kilifi, Kenya found a half-life of about two and a half months, meaning that by four to five months of age roughly half of infants were still seropositive but at declining levels.¹³ That narrow window creates a race: maternal antibodies drop below protective levels right around the age when most infants encounter RSV for the first time. Making a pediatric vaccine for this age group is complicated by several converging problems, including interference from whatever maternal antibody remains and a strong Th2 immune bias in young infants that skews the response away from effective virus neutralization.¹⁴

This is why recent prevention strategies have taken two indirect routes: vaccinating pregnant women to boost the antibodies they transfer before birth, and giving infants a long-acting monoclonal antibody that does not depend on the baby’s own immune system at all. Nirsevimab, a monoclonal antibody targeting the conserved prefusion site on the F protein, was engineered with an extended half-life so that a single dose provides protection for an entire RSV season, overcoming the need for the monthly injections that its predecessor palivizumab required.¹⁵

Diagnosing RSV in Practice

Clinicians have several layers of diagnostic tools for RSV, each with trade-offs between speed, accuracy, and cost. Rapid antigen detection tests (RADTs) give results at the bedside in minutes and are widely used in pediatric emergency departments. A systematic review and meta-analysis pooling data across many studies found that these tests achieved about 80% sensitivity and 97% specificity overall.¹⁶ In adults, however, sensitivity plummeted to roughly 29%, meaning the tests missed most adult infections. This poor performance in adults is partly because adults shed less virus, so there is simply less antigen in the sample for the test to detect. A separate evaluation in a tertiary pediatric hospital confirmed a similar pattern in children, with sensitivity around 79% and a tendency for false negatives to cluster in samples with lower viral loads, as reflected by higher cycle-threshold values on the reference test.¹⁷

RT-PCR, which amplifies and detects viral genetic material, is the gold standard for sensitivity. In children, one study comparing RT-PCR against fluorescent antibody testing found sensitivity of about 99% and specificity of about 96%, with all discordant cases being samples that were positive only by PCR and confirmed on repeat testing.¹⁸ In adults, RT-PCR detected RSV in about 73% of confirmed infection episodes, compared with only 39% by viral culture, underscoring how much more sensitive molecular methods are than traditional approaches.¹⁹ The trade-off has historically been turnaround time: conventional RT-PCR requires a laboratory and several hours.

Point-of-care molecular tests are closing that gap. A systematic review identified 39 distinct point-of-care nucleic acid amplification test platforms evaluated across 74 studies, including platforms based on RT-PCR, loop-mediated isothermal amplification (LAMP), transcription-mediated amplification, and newer approaches like CRISPR-based detection.²⁰ Some of these devices are now small enough to sit on a clinic desk. One recently developed system uses RT-LAMP in a disposable microfluidic cartridge paired with a portable optical reader and a smartphone app, simultaneously testing for RSV and three other respiratory viruses in under 30 minutes, with no separate RNA extraction step required.²¹ Another centrifugal-disc-based platform achieved simultaneous detection of seven respiratory viruses, including both RSV subtypes, on a fully integrated chip.²²

Neutralization Assays and What They Measure

Beyond diagnosing active infections, researchers need ways to measure how well a person’s blood can actually block the virus. Neutralization assays do exactly this. A serum sample is mixed with live RSV and then applied to cells in a dish; if the serum contains enough effective antibodies, fewer cells get infected. These assays measure the functional ability of antibodies to prevent infection, unlike standard binding assays that only show whether antibodies stick to a viral protein without testing whether they actually neutralize the virus.²³ The two most common formats are plaque reduction neutralization assays and microneutralization assays. A detailed validation process for a high-throughput microneutralization assay demonstrated that it could reliably detect and quantify neutralizing antibodies against both RSV subtypes A and B, making it a practical tool for evaluating vaccine candidates and tracking population immunity.²⁴

The Formalin-Inactivated Vaccine Disaster and Its Legacy

No discussion of RSV immunology is complete without the 1960s vaccine tragedy that shaped all subsequent research. A formalin-inactivated RSV vaccine was given to infants and appeared safe at first, but when those vaccinated children encountered natural RSV, many became severely ill, and two died. The vaccine had induced strong antibody binding but poor neutralizing activity. When natural infection occurred, the primed immune system overreacted with an intense inflammatory response in the lungs.²⁵ Animal models pointed to Th2-skewed T cell responses as a central driver: cells producing IL-4 and IL-5 recruited waves of eosinophils into the lungs, causing tissue damage far beyond what natural infection alone would produce.

Later work showed that distinct subsets of CD4+ T cells controlled different aspects of this enhanced disease. The Th2-biased response drove airway hyperreactivity and mucus overproduction, while the Th1-associated cytokine TNF-alpha was responsible for airway obstruction and weight loss.²⁶ The formalin treatment itself turned out to be part of the problem: formaldehyde creates reactive chemical groups on proteins that redirect the immune system toward a Th2 response, an effect that could be partially reversed by chemically reducing those groups.²⁷ This catastrophe made the field intensely cautious. It took decades before RSV vaccine candidates reached clinical trials again, and modern designs have gone to great lengths to present the F protein in its prefusion shape to elicit the right kind of neutralizing antibody without the dangerous Th2 skew.

RSV and the Road to Asthma

A persistent question in pediatric medicine is whether severe RSV infection in infancy contributes to childhood asthma. RSV is the main cause of bronchiolitis in babies, and one widely cited estimate puts the rate of subsequent childhood asthma at about 40% among children with a history of bronchiolitis.²⁸ Whether this represents a causal relationship or a shared susceptibility remains debated, but early-life RSV lower respiratory infections are increasingly recognized as a risk factor for recurrent wheezing and asthma-like symptoms.²⁹ Large population-based birth cohort studies have attempted to tease apart the relationship, though establishing clear causation has remained elusive.³⁰ One possibility is that RSV damages developing airways at a critical period, setting up long-term changes in airway reactivity. Another is that children genetically predisposed to asthma are simply more likely to have severe RSV episodes. The two explanations are not mutually exclusive, and both probably contribute.

Two Subtypes Evolving in Parallel

RSV comes in two antigenic subtypes, A and B, which co-circulate and often alternate in dominance from season to season. Genomic analysis of their evolution shows that while both subtypes mutate at similar rates, they have followed different demographic histories. RSV-A shows greater overall genetic diversity and has a more recent common ancestor estimated at about 47 years before the analysis, while RSV-B’s most recent common ancestor was only about 19 years back, suggesting it went through a genetic bottleneck that narrowed its diversity.³¹ Genotype prevalence has shifted over time. For RSV-A, genotypes like GA5 dominated for years but disappeared after 2015, with NA1 becoming the sole prevalent genotype. For RSV-B, the BA genotype family has dominated for the past two decades, with BA9 the only prevalent genotype since 2015.³² These shifts matter for diagnostics and vaccines: a test or vaccine designed around a genotype that is no longer circulating could lose effectiveness.

The Nasal Microbiome as a Disease Modifier

One of the more surprising findings in recent RSV research is that the bacterial community already living in an infant’s nose appears to influence how sick the baby gets. A multicenter prospective study tracked the nasal microbiota of RSV-infected infants and found that specific bacterial clusters were strongly enriched in severe cases. Clusters likely containing Streptococcus pneumoniae and Haemophilus species were associated with dramatically higher odds of severe disease. In contrast, bacteria associated with healthy airways, particularly Corynebacterium and Dolosigranulum species, became progressively more depleted as disease severity increased.³³ Whether these microbial shifts drive severity or simply reflect it is still being worked out, but the findings raise the intriguing possibility that supporting a healthy nasal microbiome could one day become part of RSV management, alongside vaccines and antivirals.

New Antivirals Targeting the F Protein

Beyond monoclonal antibodies for prevention, researchers are developing small-molecule drugs that directly block RSV entry. Most of these compounds target the F protein’s prefusion-to-postfusion transition, jamming the mechanism that drives membrane fusion. One challenge is that mutations in the F protein can confer drug resistance. A variant called K394R, for example, reduces the effectiveness of some fusion inhibitors. Recent work identified a compound that inhibits RSV entry through a different mechanism altogether: it blocks the interaction between the viral F protein and a host receptor called IGF1R, sidestepping K394R-associated resistance entirely.³⁴ Finding drugs that target different steps of the same process gives clinicians potential combination strategies to reduce the odds of resistance emerging, a principle borrowed from the treatment of HIV and hepatitis C.

Meanwhile, mRNA vaccine technology, accelerated into public consciousness by COVID-19, is being applied to RSV. In animal models, mRNA vaccines encoding either prefusion-stabilized or native forms of the F protein elicited strong neutralizing antibody responses comparable to those seen with protein-based vaccines, but with an added advantage: robust CD4+ and CD8+ T cell responses that the protein-based vaccine did not produce at similar doses.³⁵ Given the importance of T cells in clearing RSV, even as they carry the risk of immunopathology described above, calibrating this cellular arm of the response will be one of the defining challenges for next-generation RSV vaccines.