Influenza

H1N1 has two surface glycoproteins: hemagglutinin and neuraminidase. Hemagglutinin mediates viral attachment by binding to sialic acid receptors on respiratory epithelial cells, while neuraminidase cleaves sialic acid residues to release progeny virions. Both proteins are essential for viral infectivity, transmission, and immune recognition.

Hemagglutinin binds to sialic acid receptors on respiratory cells, triggering viral internalization into endosomes. Acidification within the endosome causes hemagglutinin to fuse the viral envelope with the endosomal membrane, releasing viral ribonucleoprotein complexes into the cytoplasm. Host importin proteins then recognize nuclear localization signals and transport these complexes through nuclear pore complexes into the nucleus.

Influenza is unusual among RNA viruses because replication occurs in the host cell nucleus rather than the cytoplasm. Viral RNA transcription and genome replication take place in the nucleus after viral ribonucleoprotein complexes are transported there by importin proteins, distinguishing influenza from most other RNA viruses.

Neuraminidase cleaves residual sialic acid residues from the host cell surface after viral assembly and budding occur at the plasma membrane. This enzymatic activity is required to release progeny virions from the cell surface, completing the viral life cycle and enabling infection of surrounding cells and rapid spread within the respiratory tract.

The low pH inside the endosome triggers a conformational change in hemagglutinin, promoting fusion of the viral envelope with the endosomal membrane. This fusion releases viral ribonucleoprotein complexes into the cytoplasm, allowing viral RNA segments to proceed to the nucleus for replication and initiating the next stage of the viral life cycle.

Newly synthesized viral mRNA is exported to the cytoplasm for translation into viral proteins. Both viral proteins and replicated RNA segments are then transported to the plasma membrane, where assembly and budding of progeny virions occur, preparing them for release and infection of neighboring cells.

Influenza is an acute, highly communicable viral disease affecting the respiratory tract, transmitted primarily through respiratory droplets and contaminated surfaces. The rapid spread of influenza within the respiratory tract occurs because neuraminidase facilitates progeny virion release, enabling successive infection of neighboring respiratory epithelial cells, similar to mechanisms seen in respiratory syncytial virus disease.