Influenza A [M2110169]

Influenza virus is a type of enveloped, segmented, negative-sense, single-stranded RNA virus of the Orthomyxoviridae family. There are three major antigenic types of influenza virus that are clinically relevant to humans, including Flu A, B and C. Flu A viruses affect humans and bird populations, whilst Flu B and C only infect humans. Based on the antigenicity of the glycoproteins, influenza A viruses are subdivided into sixteen H (H1-H16) and nine N (N1-N9) subtypes. The main antigenic determinants of influenza A and B viruses are the hemagglutinin (HA) and neuraminidase (NA) transmembrane glycoproteins. Projections of HA and NA cover the surface of the virus particle. NA forms a tetramer with an average molecular weight of 220 kDa (~55 kDa per monomer). The matrix (M) protein of influenza A virus is one of the two group-specific internal proteins of the virion, The non-structural protein (NP) exists as a homeodimer (molecular weight of 52 kDa) consisting of two identical monomers (each ~26 kDa). Flu A viruses are further divided into subtypes based on the expression and combination of two envelope glycoproteins called haemagglutinin (H, also referred to as HA) and neuraminidase (N), which are important targets for the immune system. Flu A viruses undergo high rates of mutation and gene rearrangement which lead to antigenic variations of these glycoproteins. There are 18 H subtypes and 11 N subtypes recognized to date. The subtypes currently circulating among the human population are Flu A (H1N1) and Flu A (H3N2). In most healthy individuals, Flu A causes a self-limiting respiratory illness, but the virus can cause severe illness or death in the elderly and high-risk patients. Transmission of the virus between humans is predominantly the result of contact with contaminated airborne droplets, from an infected individual which are inhaled and enter the respiratory tract, but can also be spread by contaminated hands or surfaces.