- Influenza virus.

- Influenza virus belongs to Orthomyxoviridae family (single-stranded, negative-sense RNA,enveloped viruses and replicate by nuclear replication).

- Genera which are identified by antigenic differences in their nucleoprotein and matrix protein :-

• Influenza A virus.
• Influenza B virus.
• Influenza C virus.

*Influenza Virus Morphology*

- The particles are mostly spherical, 80-120 nm diameter,its core diameter 9 nm and has helical nucleoprotein.

-Nucleoprotein(RNA + nucleoprotein, NP)is any protein which is structurally associated with nucleic acid.

-Matrix proteins(M1,M2)are structural proteins linking the viral envelope with the virus core.

- The virus has a lipid envelope from which project 500 prominent glycoprotein spikes 10 - 14 nm from the surface and are of two types:-

1. Haemagglutinin (HA or H)"the major" which is a sugar-binding protein that mediates binding of the virus to target cells and entry of the viral genome into the target cell.

2. Neuraminidase (NA or N) which is an enzyme involved in the release of virus progeny from infected cells, by cleaving sugars that bind the mature viral particles.

-Ratio of HA to NA is about 4-5 to 1.

(Simplified cartoon showing structure of influenza virus image from Nature)

*Virus Mutation*

-Every 10 - 15 years a major new pandemic strain appears in man, with a totally new H and sometimes a new N as well (Antigenic Shift) which means complete change in HA and NA.

-This new variant may cause a major epidemic around the world.

- Over the subsequent years this strain undergoes minor changes (Antigenic Drift) every two to three years which means incomplete change in HA and NA.

*serotypes:-

- Influenza A viruses are further classified, based on the viral surface proteins (HA or H) and (NA or N) into 16 H subtypes (or serotypes) and 9 N subtypes giving a large number of subtypes due to different associations.

-Influenza B: Known only in man and seals and is less common than influenza A Undergoes relatively slow change in HA with time,consequently is less genetically diverse, with only one influenza B serotype as a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age.

-Influenza C: Uncommon strain, known only in man and pigs and can cause severe illness and local epidemics.

-Influenza A viruses have an importance nowadays due to lack of specificity to a single host tissue and genome re-assortment which leaded to a panic in the world as for example human Influenza A H1N1 was causing seasonal flu in man and Swine Influenza A H1N1 was causing endemic flu in pigs,swine H1N1 made re-assortment in pig to be transmitted to man leading to pandemic of swine flu in world.

Dr Ibrahim...