Antigenic Drift

Antigenic Drift is a mutation in the genetic code in the surface antigens. The mutation occurs in the Haemagglutinin (HA) and Neuraminidase (NA) on the surface of the virus. The changes in the antigens allow influenza to evade the immune systems of the host and make a yearly vaccination a necessity. The scientific nomenclature of influenza comes from the number HA or NA spikes on the surface of the virus. The influenza occurs in three subtypes: A, B and C. Type A and B are the most virulent strain and type C causes mild infections. Type A infects humans, birds and other mammals, while B and C only infect humans. Type A and B are responsible for all epidemics, but type A has a greater likelihood. For example, A(H1N1) would be influenza type A with one HA spike and one NA spike. Currently, there are three subtypes circulating, two of which are classified as type A and one type B.

Another way influenza evades the immune system of the host is through antigenic shift. Antigenic shift occurs when large fragments of genetic material are replaced with genes from other influenza subtypes. It occurs during reassortment, when the genetic material of the virus is being packed into a new viron used to infect other cells. Only influenza A can change by antigenic shift and is responsible for world pandemics and high mortality rates.

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email: David Zage

Last modified: 4/24/98