Cell Invasion
The first step of influenza's replication process is invading the cell. Influenza attaches itself to epithelial cells (cells that line tissues), usually in the lungs, nose and throat. The influenza virus attaches itself to these cells using a protein called hemagglutinin. The hemagglutinin causes the cell to think that the virus is something that needs to be brought inside, so the cell will cut off the hemagglutinin using a protein and then bring the virus inside via a process called endocytosis. With endocytosis, the cell takes a little bit of its outside membrane and makes a little pocket that it uses to pull the virus inside.
Intracellular Replication
Once the virus is inside the cell, a number of things happen. First, the virus creates a small channel so that it can escape from the pocket the cell has made for it. Next, the virus core disassembles once it is inside the cell. This allows the virus's genetic information to be released into the cell, as well as proteins that allow it to replicate. The virus's genetic material and replication proteins then go into the nucleus of the cell, where the virus will then use the cell's machinery to make copies of itself. The virus continues to replicate in the cell. Some of the copies will then leave the cell, taking small bits of the cell's membrane as it leaves.
Antigenic Drift
Viruses use their own proteins to make copies of their genetic information. However, these proteins are prone to making mistakes. This causes small changes or mutations to appear in the virus's genetic information. These genetic mutations cause something called "antigenic drift." Antigenic drift causes small variations to appear in copies of the influenza virus. This is why a new "flu vaccine" is needed each year, to account for changes in the viruses that cause influenza.
