Paper II — 2014 September (Supplementary) (2010 Scheme) — Q13
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Paper II
2014 September (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Morphology of influenza virus
Q132 marksShort Notes
Answer
Influenza virus (family Orthomyxoviridae) is a spherical to pleomorphic, enveloped, single-stranded, negative-sense, segmented RNA virus, with a genome divided into 8 separate gene segments (for influenza A and B) — this segmented genome structure is functionally critical, as it permits genetic reassortment between different strains co-infecting a single cell, the mechanism underlying antigenic shift.
Structure:
Envelope — a lipid bilayer derived from the host cell membrane during budding, studded with two major glycoprotein spikes:
Haemagglutinin (HA) — mediates attachment of the virus to sialic acid receptors on host respiratory epithelial cells and facilitates membrane fusion for viral entry; the primary target of neutralizing antibody and the basis for the “H” in strain nomenclature (e.g., H1N1, H3N2).
Neuraminidase (NA) — an enzyme that cleaves sialic acid, facilitating release of newly formed virions from the infected cell surface (preventing self-aggregation) and aiding spread through respiratory mucus; the target of neuraminidase inhibitor antiviral drugs (oseltamivir, zanamivir); the basis for the “N” in strain nomenclature.
Matrix (M1) protein — lies beneath the envelope, providing structural support.
Nucleocapsid — a helical ribonucleoprotein complex, each of the 8 RNA segments individually encapsidated by nucleoprotein (NP) and associated with the viral RNA-dependent RNA polymerase complex.
Classification: divided into types A, B, and C (and a less common type D) based on antigenic differences in the internal nucleoprotein and matrix protein — influenza A is further subtyped by its HA and NA combinations (18 HA and 11 NA subtypes recognized), and is the only type capable of undergoing antigenic shift (via reassortment with animal-reservoir strains), making it responsible for pandemics, while both A and B undergo the more gradual antigenic drift (point mutations), responsible for seasonal epidemics and the need for annual vaccine reformulation.
