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Structure of Influenza B Virus

There are three types of influenza viruses: A, B and C. Types A and B are structurally similar. Type C is genetically different and non-symptomatic, so it receives little medical attention. These influenza viruses are from the RNA family called Orthomyxoviridae. Despite similarities between types A and B, they behave quite differently.

  1. Structure

    • The overall size and shape of the influenza virus varies very little. The diameter ranges from 80 to 120 nanometers. The virus sits in a lipid bilayer that is derived from the host. Two forms of glycoprotein, hemagglutinin and neuraminidase, poke through the outside of the structure. They assist in attaching the virus to the host and distributing new virus particles into the cells of the host after attachment. The underside of the lipid bilayer is an antigenic protein lining determining the type of influenza. Finally, at the center is the genome itself are eight pieces of single-stranded RNA. There are three polymerase peptides for each segment of RNA.

    Hemagglutinin

    • Hemagglutinin is so named because it is able to agglutinate, or clump together, the host's red blood cells. It is composed of two chains spiraled together that act to locate the sugar chains in the cellular proteins of the host so the virus can latch on and transfer the viral genome. There are many subtypes of hemagglutinin; those that bind to humans are H1, H2 and H3. The morbidity and mortality rate of the influenza strain is determined by which hemagglutinin it carries.

    Neuraminidase

    • Neuraminidase is shaped like a square, attaching itself to the virus with a protein stalk. There are nine types of neuraminidase: N1 through N9. This variety of subtypes assists the flu virus in being effective. Neuraminidase has the job of ensuring that the flu virus does not stick to the host cell. If the polysaccharide chains catch, the neuraminidase clips them free.

    Behavior

    • Type A influenza affects many different species of animals and mutates rapidly; most influenza pandemics are caused by variations of type A. Influenza B can be acquired only by humans and seals. It mutates very slowly, making it easy for scientists to monitor its mutations and vaccinate against each new strain. Both A and B are responsible for localized epidemics of seasonal flu. When you get a flu shot, you are being vaccinated against the current strains of types A and B influenza.

    Mutation

    • Mutation occurs naturally in evolution, viruses included. In influenza, it is a distinct change in the viral RNA. These seemingly slight changes make the entire virus unrecognizable to the immune system. Antigenic drift is a gradual change in the virus; antigenic shift is an abrupt change. Influenza B undergoes only antigenic drift.

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