![]() ![]() 1 Antigenic shift happens as a result of mixing of the genome pieces from several different viruses that infected the same host cell. Unlike antigenic drift, antigenic shift occurs at unpredictable intervals, when an existing influenza virus disappears and is replaced by a new subtype with new surface proteins. Therefore, influenza vaccines must be reformulated continuously to keep pace with the constant mutations of the flu virus. Point mutations that involve coding for the surface proteins H and N can change the proteins enough that they no longer are recognized by the immune system. These changes, called point mutations, are common in RNA viruses, such as influenza. Many of these of mutations occur as a random change in a single nucleotide. 5 Antigenic drift is defined as the random accumulation of mutations in all the virus genes, including those that code for the surface proteins. It does so via two main mechanisms: antigenic drift and antigenic shift. The influenza virus continuously changes to evade its host's acquired immunity. Slight changes in these antigenic proteins allow the virus to escape detection by the immune system. Foreign molecules, such as these proteins, that stimulate an immune response, are called antigens. The host immune system identifies and remembers invading influenza viruses primarily based on characteristics of their surface proteins, hemagglutinin and neuraminidase. Currently, subtypes H1N1 and H3N2 are responsible for outbreaks of human flu. In addition, since the H and N subtypes exist on different RNA segments, a host infected with different viral genotypes can produce new viruses with reassorted, and potentially unique, genotypes. 1Įach of the H and N subtypes reacts differently with antibodies produced by the immune system. Most of these subtypes only have been detected in birds. ![]() To date, 15 subtypes of H (H1 through H15) influenza A viruses, along with nine subtypes of N (N1 through N9) influenza A viruses, have been found. Neuraminidase is involved in the release of new virus particles from host cells. Hemagglutinin has a role in attachment of the virus to cells in the respiratory system. Both of these proteins are present as spikes on the outer surface of the virus. Influenza A viruses are further divided into subtypes, based on characteristics of their surface proteins, hemagglutinin (H) and neuraminidase (N). Influenza A and B viruses are the two types that lead to flu in humans. Type C infection produces only mild respiratory symptoms and has not caused significant disease. The other two types, B and C, are found primarily in humans. Type A influenza, the most common form, is found in a wide variety of birds and mammals. There are three types of influenza virus-A, B and C-which are distinguished by differences in two of their internal proteins (nucleoprotein and matrix protein), differences in their pathogenicity (degree to which they cause disease), and differences in their organization of genetic information (influenza virus types A and B have eight RNA segments type C has seven). Like all viruses, influenza must invade a host cell and hijack its machinery to manufacture more viruses. The genetic information of influenza virus is encoded as single stranded RNA. Each year, it causes illness and death in millions of people globally. The influenza virus, a member of the family Orthomyxoviridae (Greek myxa, means "mucus"), 1 is one of the world’s most important and dangerous respiratory pathogens. influenzae was termed Pfeiffer's Bacillus, where it was found in the sputum of many influenza patients, and thought to be the cause of influenza. Photomicrograph of Haemophilus influenzae as seen using a Gram stain technique.
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