Influenza A viruses tend to garner most of the attention, but let’s not forget that there are two other virus types, B and C.
The enveloped influenza A virions have three membrane proteins (HA, NA, M2), a matrix protein (M1) just below the lipid bilayer, a ribonucleoprotein core (consisting of 8 viral RNA segments and three proteins: PA, PB1, PB2), and the NEP/NS2 protein. It would be difficult to distinguish influenza A and B viruses by electron microscopy, but there are differences. Influenza B virions have four proteins in the envelope: HA, NA, NB, and BM2. Like the M2 protein of influenza A virus, the BM2 protein is a proton channel that is essential for the uncoating process. The NB protein is believed to be an ion channel, but it is not required for viral replication in cell culture.
Influenza B viruses cause the same spectrum of disease as influenza A. However, influenza B viruses do not cause pandemics. This property may be a consequence of the limited host range of the virus – humans and seals – which limits the generation of new strains by reassortment. The virus causes significant morbidity: in the US in 2008, approximately one-third of all laboratory confirmed cases of influenza were caused by influenza B (as shown on the first graph on this CDC page). Consequently the seasonal trivalent influenza vaccine contains an influenza B virus component.
Influenza C viruses are somewhat different (there is a nice diagram here). The enveloped virions have hexagonal structures on the surface and form long (500 microns) cordlike structures as they bud from the cell (image below). Like the influenza A and B viruses, the core of influenza C viruses consists of a ribonucleoprotein made up of viral RNA and four proteins. The M1 protein lies just below the membrane, as in influenza A and B virions. A minor viral envelope protein is CM2, which functions as an ion channel. The major influenza C virus envelope glycoprotein is called HEF (hemagglutinin-esterase-fusion) because it has the functions of both the HA and the NA. Therefore the influenza virion contains 7 RNA segments, not 8 RNAs like influenza A and B viruses.
Nearly all adults have been infected with influenza C virus, which causes mild upper respiratory tract illness. Lower respiratory tract complications are rare. There is no vaccine against influenza C virus.
I know influenza B and C viruses quite well – I did my Ph.D. research on them. I showed that the influenza C virus genome consists of 7 RNA segments, and demonstrated reassortment among different influenza C virus strains.
Hatta, M., & Kawaoka, Y. (2003). The NB Protein of Influenza B Virus Is Not Necessary for Virus Replication In Vitro Journal of Virology, 77 (10), 6050-6054 DOI: 10.1128/JVI.77.10.6050-6054.2003
Racaniello VR, & Palese P (1979). Isolation of influenza C virus recombinants. Journal of Virology, 32 (3), 1006-14 PMID: 513198