In humans, the primary targets for influenza viruses are epithelial cells in the upper and lower respiratory tract. As indicated above, the viral HA binds to sialic acid residues on glycoproteins or glycolipids on the cell surface. 9 The fine specificity of HA's receptor binding depends on the nature of the glycosidic linkage between the terminal sialic acid and the penultimate galactose residue on the receptor. 22 Human influenza viruses preferentially bind to sialic acids attached to galactose in an α2,6 configuration, whereas avian viruses have a preference for sialic acids attached to galactose in an α2,3 linkage. 23 This difference is thought to be the basis for the very inefficient transmission of avian influenza viruses to humans. Pigs, on the other hand, have receptors with either type of linkage between sialic acid and galactose, and thus are readily susceptible to infection with both human and avian viruses. 23 Co-infection of pigs with different influenza viruses is considered one mechanism by which new influenza viruses with pandemic potential may arise (see also Chapter 3).Receptor binding initiates uptake of the virus through so-called receptor-mediated endocytosis. In this process, virus particles are engulfed by the host cell plasma membrane ( Figure 8 ). The vesicles thus formed subsequently fuse with intracellular compartments called endosomes, as a result of which the virus is delivered to the endosomal lumen.24, 25, and 26 Receptor-mediated endocytosis is not specific for uptake of viruses. In fact, it is a general mechanism by which cells internalize macromolecular complexes. Substances taken up by endocytosis, having traversed the endosomal cell compartment, generally end up in lysosomes, where they are degraded by hydrolytic enzymes. The influenza virus genome, however, escapes degradation; through fusion of the viral envelope with the endosomal membrane, it gains access to the cell cytosol ( Figure 8 ).
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