Antigenic Variation Among Type A Influenza Viruses

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Abstract

Influenza is a worldwide disease that periodically causes epidemics in humans, horses, pigs, birds, and occasionally in other animals such as seals. The hallmark of influenza virus is its variability, having the capacity to change its antigenic identity so remarkably that the specific immunity established in response to infection by a particular strain may give little or no protection against viruses that subsequently arise. Antigenic variation is the result of molecular changes in the surface proteins of influenza viruses, i.e., the hemagglutinin (HA) and neuraminidase (NA), which reflect alterations in the sequences of the constituent genes. There are two types of change that the HA and NA can undergo; these occur by different mechanisms and are known as antigenic “drift” and antigenic “shift”. Antigenic “drift” occurs within a subtype and involves a series of minor changes at the gene level (usually point mutations) so that variants are formed, each slightly different from its predecessor. On the other hand, antigenic “shift” is caused by a more radical change in the HA and/or NA. Here influenza viruses appear in the population with surface antigens unlike those of viruses immediately preceding them. The origin of these new viruses is still unclear; some may result from genetic reassortment between human and animal virus strains and others may exist in a dormant stage for long periods before reemerging.