The influenza, or flu, virus is a common respiratory pathogen responsible for seasonal outbreaks of illness. This virus primarily targets cells in the nose, throat, and lungs. This article will explore the structure of the flu virus, its infection and replication processes, how it changes over time, and the ways it leads to the characteristic symptoms of influenza.
The Flu Virus Structure
The influenza virus is spherical. Its outer layer is a lipid membrane, known as an envelope, which it acquires from the host cell it infects. Embedded within this envelope are numerous spike-like proteins, primarily Hemagglutinin (HA) and Neuraminidase (NA). These proteins are important for the virus’s ability to interact with and infect host cells.
Inside this envelope, beneath a viral protein shell called M1, lies the virus’s genetic material. This material consists of eight segments of single-stranded RNA for influenza A and B viruses. Each RNA segment is associated with proteins, forming a ribonucleoprotein (RNP) complex.
How the Virus Infects and Replicates
The infection process begins when the flu virus enters the respiratory tract, often through airborne droplets from a cough or sneeze. The Hemagglutinin (HA) proteins on the viral surface bind to specific sugar molecules called sialic acid receptors found on the surface of host cells in the respiratory system. This binding is a necessary first step for the virus to gain entry.
After attachment, the host cell engulfs the virus through a process called endocytosis, creating a bubble around it. Inside this bubble, the acidic environment triggers changes in the HA protein, leading to the fusion of the viral envelope with the bubble’s membrane. This fusion releases the viral genetic material, the RNA segments, into the host cell’s cytoplasm.
The viral RNA segments, along with associated proteins, then travel into the host cell’s nucleus. Here, the viral RNA polymerase uses the viral RNA as a template to create new copies of viral RNA and messenger RNA (mRNA). The host cell’s machinery then translates this mRNA into viral proteins, effectively hijacking the cell’s resources.
New viral RNA segments and proteins are then assembled into progeny viruses. These newly formed viruses move to the cell surface, where they bud off. The Neuraminidase (NA) protein helps cleave sialic acid receptors on the host cell, allowing the newly formed viruses to detach and spread.
How the Virus Changes Over Time
Influenza viruses constantly evolve, which explains why new strains emerge and why annual vaccination is often recommended. One way the virus changes is through antigenic drift, involving small, continuous genetic mutations in the HA and NA genes during replication. These minor changes accumulate over time, leading to slightly altered surface proteins that the immune system may no longer fully recognize.
Antigenic drift can allow the virus to evade existing antibodies from previous infections or vaccinations, making individuals susceptible to influenza again. This gradual change is the primary reason why the flu vaccine composition needs regular updates.
A more dramatic change occurs through antigenic shift, which involves an abrupt, major alteration in the HA and/or NA proteins. This typically happens when two different influenza A virus strains co-infect the same host cell, leading to a reassortment of their genetic segments. The result is a novel subtype of the virus with a new combination of surface proteins, to which most of the human population has little or no pre-existing immunity.
Antigenic shifts can lead to widespread outbreaks or pandemics because the new virus can spread rapidly through a largely unprotected population. While antigenic drift happens continually, antigenic shift occurs less frequently and only affects influenza A viruses.
How the Flu Causes Symptoms
The symptoms associated with influenza primarily result from the body’s immune response to the viral infection and direct damage to infected cells. When the virus infects cells in the respiratory tract, the immune system detects its presence and mounts a defense. This defense mechanism involves releasing signaling molecules, such as cytokines, which trigger a systemic inflammatory response.
This inflammatory response contributes to many common flu symptoms like fever, muscle aches, and fatigue. The immune system’s efforts to clear the virus also lead to localized inflammation in the respiratory passages, causing symptoms such as sore throat, cough, and a runny or stuffy nose.
The virus directly damages the epithelial cells lining the respiratory tract as it replicates and spreads. This cellular destruction further contributes to respiratory symptoms and can impair the body’s natural defenses, potentially leading to secondary bacterial infections. The severity of symptoms can vary widely depending on the specific viral strain, an individual’s immune status, and any underlying health conditions.