Classification of influenza virus antigens
The entire antigenic structure of the viral particle can be divided into two large groups depending on the precise localization of the proteins and their role in classifying the infectious agent.
1. Internal antigens These structures are concealed inside the virion. They include the nucleoprotein (NP protein) and matrix proteins (M proteins). Their key feature is high stability and genus specificity. It is based on internal antigens that the viral type is determined: A, B, or C. Complement fixation test (CFT) and enzyme-linked immunosorbent assay (ELISA) are traditionally used in laboratory diagnostics to detect internal proteins.
2. Surface antigens Two major glycoproteins are located on the viral envelope: hemagglutinin (HA) and neuraminidase (NA). They act as protective antigens, meaning that the human immune system begins producing protective antibodies specifically in response to their introduction. These proteins determine the specific subtype and variant of the virion; importantly, HA and NA can mutate completely independently of each other. A vast diversity of these molecules exists in nature: 15 variants of hemagglutinin and 9 variants of neuraminidase have been identified. However, stable pathogenicity in humans is demonstrated only by combinations with H1, H2, H3, as well as N1 and N2. Surface structures are detected using the hemagglutination inhibition assay (HIA) and ELISA.
Antigenic drift: a strategy of small steps
Influenza A virus is characterized by striking genetic flexibility. One of its two pathways of transformation is called antigenic drift (Drift).
The essence of this process lies in gradual, extremely minor structural changes in surface molecules. At the genetic level, this is driven by point mutations in the genomic regions encoding the antigenic determinants of HA and NA.
Features of drift:
- Occurs frequently and continuously, background-accompanying the pathogen's circulation.
- Leads to the formation of new serovariants of the virus. They differ from the parental strain while maintaining a genetic relationship with it.
- Is the cause of regular epidemics that flare up every 2–3 years.
Under conditions of drift, previously established immunity in recovered individuals provides only partial protection. Furthermore, accumulated herd immunity acts as the main factor of natural selection: only those mutants that have learned to bypass existing population defense survive and spread.
Antigenic shift: a global threat
The second, much more dangerous mechanism of variability is antigenic shift (Shift). This is a radical transformation involving the complete replacement of one or both surface proteins (hemagglutinin or neuraminidase).
Such a shift is made possible by genetic recombination, or reassortment. If two completely different viral subtypes infect the same host cell simultaneously, they can exchange genetic segments. Animal influenza strains frequently participate in this process by mixing with human variants.
Unlike drift, shift occurs rarely. However, its result is the emergence of a fundamentally new viral subtype. The epidemiological significance of such an event is enormous: since the human population has no specific immunity to the "novelty" whatsoever, local outbreaks rapidly escalate into global pandemics.
Briefly: answers to topical questions
What biological functions does hemagglutinin (HA) perform during interaction with the host cell?
Hemagglutinin (HA) ensures the primary attachment of the virus and its entry into the susceptible cell.
- Adhesion — binds to specific receptors on the plasma membrane of target cells (sialic acid residues, α-2', 6' galactose-containing oligosaccharides).
- Penetration — participates in the formation of the endocytic vacuole and binds to endosomal proteins and membrane lipids, ensuring the introduction of the virus into the cytoplasm via receptor-mediated endocytosis.
What biological functions does neuraminidase (NA) perform in the life cycle of the influenza virus?
Neuraminidase (NA) performs an enzymatic function by cleaving sialic acid and facilitating virus dissemination.
- Overcoming barriers — thins respiratory tract mucus by neutralizing sialic acid in mucins, facilitating viral access to the epithelium.
- Penetration — modifies cellular receptors, facilitating viral entry into the cell via endocytosis.
- Release of virions — disrupts the bonds between hemagglutinin and sialic acids on the cell surface, ensuring the final budding of new viral particles and preventing their aggregation.
Which types of influenza virus (A, B, C) are capable of undergoing antigenic shift?
Only influenza A virus is capable of undergoing antigenic shift. This process represents a radical structural change of surface antigens (complete replacement of hemagglutinin or neuraminidase) as a result of genetic recombination (reassortment). Shift occurs upon simultaneous infection of a single cell by different viral subtypes, leading to the exchange of entire RNA segments and the formation of a fundamentally new subtype that causes pandemics.
Which animal species act as the primary reservoir for the generation of shifted influenza virus variants?
Animals, especially birds, serve as the primary natural reservoir of antigens for the formation of new shifted variants of influenza A virus.
Swine participated in the generation of the 2009 pandemic strain: it emerged via the reassortment of two swine influenza A genotypes and combined genetic material from swine, avian, and human viruses.
To avoid confusing the mechanisms: Drift is like a slow river current (minor, frequent point mutations, epidemics every 2–3 years). Shift is like pressing the Shift key on a keyboard: a sharp, complete register switch (complete antigen replacement, new subtypes, and global pandemics).
Class and exam questions
Which antigens determine the type of influenza virus (A, B, or C)?
The viral type depends on internal genus-specific antigens — nucleoprotein (NP) and matrix proteins (M proteins).
Why do regular seasonal influenza epidemics occur?
Antigenic drift is the cause. Constant point mutations of surface proteins lead to the emergence of new serovariants, against which pre-existing herd immunity provides only partial protection.
How does antigenic shift differ from drift?
Shift is a radical, complete replacement of hemagglutinin or neuraminidase through gene exchange between different viruses in a single cell. This is a rare event leading to pandemics, whereas drift occurs continuously.
What else to study on this topic
- Methods of laboratory diagnostics: CFT, ELISA, and HIA.
- Structural features and functions of neuraminidase and hemagglutinin.
- The role of herd immunity as a factor in viral evolutionary selection.
- The mechanism of genetic reassortment at the cellular level.
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