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What is influenza or flu? Influenza A, B and C; function of hemaglutinin and neuraminidase, and flu virus nomenclature
Influenza A, B and C.
Influenzavirus A belongs to the Orthomyxoviridae family of viruses. Wild aquatic birds are the natural hosts for different strains of influenza A. However, humans can get infected ocassionally. The type A infection in the human can be very severe and have caused pandemic deaths in the past. Influenza B infects mostly human but is less common and Influenza C which may infect human, pigs and dogs is less common than A and B. All the three, influenza A, B an C belong to the same family of viruses, but different genera.
The overall structure of all influenza viruses is the same. In technical terms, Influenza A virus is a single stranded RNA virus. The RNA genome of influenza A virus is anti-sense and segmented. The eight RNA segments in influenza A virus code for eleven proteins. Though all the eleven proteins are important for the survival and propagation of the virus, the two critical proteins that are important in the context of its transmission and nomenclature are HA and NA. HA stands for Hemaglutinin A and NA stands for Neuraminidase.
What is hemaglutinin A and its function?
The function of HA is in determining the infection of the host cells. HA on the surface of the viral particles binds to certain very specific sugar molecules on the surface of target host cells. This allows the virus to stick to the surface of host cells. Now that the virus is attached to host cell, the host cell takes it inside by a process called endocytosis. Once inside the cell, the virus again uses the same HA protein as an anchor to get out of the endocytotic vescicles to get into the cytoplasm.
What is neuraminidase and its function?
Once inside the cell, the viral RNA uses the host cell's machinery to multiply itself. Once again, to exit the host cell, the virus needs to cleave certain specific sugar residues on its own surface as well as on the host cell to detach itself. This is done by the enzyme (an enzyme is a biological catalyst), neuraminidase, or NA.
Naming the virus: basis of Influenza virus A nomenclature.
Hemaglutinin and neurominidase form the outer coat of the viral particles with the core of the particle formed with genomic RNA and other proteins. There are 16 subtypes of HA protein and 9 subtypes of NA proteins. That means there can be 144 different combinations of influenza A viruses. These protein subtypes are named H1-H16 and N1-N9 and viruses carrying the specific combinations of these proteins are named according these subtypes, for example a virus with H1 form of HA and N1 for NA is called H1N1.