Virus Infection
Replication of the influenza virus requires multiple steps and processes. The first part is the the mechanism by which the virus is able to infect cells. Influenza viruses typically infect epithelial cells (which are thin flat cells) that line the throat, nose and lungs of mammals and the intestines of birds. Flu viruses have a protein called hemagglutinin on their surface, which resembles a protein found normally in the body. The virus uses these hemagglutinin proteins to bind to a compound on the surface of epithelial cells called sialic acid, which causes the cell to import the virus via a process called endocytosis.
Virus Activation
Once the virus has been endocytosed, it is transported to a portion of the epithelial cell called the endosome. The endosome is the receiving station for newly absorbed molecules, and it is somewhat acidic. One kind of viral protein, called the M2 ion channel, allows some of the endosome acidity to get inside the virus's protective membrane (the M2 ion channel is inhibited by some anti-flu medications). This causes the core of the virus to disassemble and allows the virus to release its genetic material (RNA) and also some of its core proteins. These viral proteins then bind to the viral RNA and both are transported to the nucleus, which is where the cell keeps its DNA.
Replication and Release
Once the virus is in the nucleus, it begins making copies of itself. Some of these copies are sent out of the nucleus to be made into viral proteins, which can either prepare the cell membrane to be turned into a virus membrane (with hemagglutinin proteins, for example), while other are used to speed the virus's replication process. When the virus has made enough copies, it take one copy of its RNA and its core proteins and packages them into a complex called a virion. The virion then leaves the cell, taking a little bit of the cell's membrane as its own, using a protein called neuraminidase to leave the cell (neuraminidase is another target of some anti-flu medications).
