Influenza viruses are remarkable because of the frequent antigenic changes that occur in HA and NA. Antigenic variants of influenza virus have a selective advantage over the parental virus in the presence of antibody directed against the original strain. This phenomenon is responsible for the unique epidemiologic features of influenza. Other respiratory tract agents do not display significant antigenic variation.

The two surface antigens of influenza undergo antigenic variation independent of each other. Minor antigenic changes are termed antigenic drift; major antigenic changes in HA or NA, called antigenic shift, result in the appearance of a new subtype (Figure 1). Antigenic shift is most likely to result in an epidemic.

Fig1. Antigenic drift and antigenic shift account for antigenic changes in the two surface glycoproteins (hemagglutinin [HA] and neuraminidase [NA]) of influenza virus. Antigenic drift is a gradual change in antigenicity caused by point mutations that affect major antigenic sites on the glycoprotein. Antigenic shift is an abrupt change caused by genetic reassortment with an unrelated strain. Changes in HA and NA occur independently. Internal proteins of the virus, such as the nucleoprotein (NP), do not undergo antigenic changes.

Antigenic drift is caused by the accumulation of point mutations in the gene, resulting in amino acid changes in the protein. Sequence changes can alter antigenic sites on the molecule such that a virion can escape recognition by the host’s immune system. The immune system does not cause the antigenic variation but rather functions as a selection force that allows new antigenic variants to expand. A variant must sustain two or more mutations before a new, epidemiologically significant strain emerges.

Antigenic shift reflects drastic changes in the sequence of a viral surface protein, caused by genetic reassortment between human, swine, and avian influenza viruses. Influenza B and C viruses do not exhibit antigenic shift because few related viruses exist in animals.

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مواضيع ذات صلة

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Influenza Virus Replication

Structure and Function of Neuraminidase

Complications of COVID-19

Clinical Features of SARS-CoV-2 and COVID-19

Modes of Transmission SARS-CoV-2 and COVID-19

Papillomaviruses

Arenaviruses

Adenoviruses

Viral Serology: Immune Status Testing

Structure and Function of Hemagglutinin

Structure and Composition of influenza viruses