Enveloped. Usually rounded but can be filamentous. The virions are 80-120 nm in diameter.



segmented ssRNA(-) linear genome, encapsidated by nucleoprotein (NP) Contains 8 segments coding for 11 proteins. Segments size range from 890 to 2,341nt. Genome total size is 13.5Kb


Viral RNA polymerase (PB1, PB2 and PA) transcribes one mRNA from each genome segment. Transcription is primed by cap snatching. mRNA are polyadenylated by the viral polymerase stuttering on a poly U track. M and NS mRNA can be alternatively spliced, giving rise to mRNA coding for BM" and NEP proteins. NB is translated by leaky scanning from the NA mRNA.



  1. Virus attaches to sialic acid receptor though HA protein and is endocytosed by clathrins in the host cell.
  2. endosome acidification induces fusion of virus membrane with the vesicle membrane; encapsidated RNA segments migrate to the nucleus.
  3. Transcription of genomic segments by the viral polymerase produces mRNAs that are capped and polyadenylated by the viral polymerase.
  4. Replication of genomic segments.
  5. High level of M1 protein induces genomes segments export from nucleus by NEP protein.
  6. Virus assembly and budding occurs at the plasma membrane.

Host-virus interaction

Innate immune response inhibition

Influenza B virus inhibits the cascade leading to production of interferon-beta by targeting different cellular proteins. NS1 inhibits host ISG15 activation as well as PKR.