When a person is infected with influenza virus, the body’s immune system kicks into action by producing proteins called antibodies.
You may ask, what is an antibody exactly? How does it battle against the virus? Well, antibodies are protein molecules produced by the body which functions to detect foreign substances (influenza virus in this case).
Once the virus is detected, there will be multiplication of the antibody. These antibodies then travel around the bloodstream to find and bind to the virus. (functioning like patrol officers). 
The figure above shows how the Y shaped antibodies bind to the proteins on the virus (blue)
Antibodies deactivate the virus by binding to the proteins on the surface of the virus. This is a very significant step carried out by our immune system to prevent viruses from binding to healthy cells and infecting them. In other words, antibodies are effective in ceasing the infection from spreading.
Not only stopping the spread of the virus, antibodies also play a role in reducing the number of viruses in the body (literally killing the viruses floating around in our body). How do they do it? The antibodies by binding to the virus aggregate the viruses rendering it useless. This aggregation is also the sign for macrophages (cells which engulfs and kills the virus) to come into action.
When these battle rages inside your body, you might be aware of the symptoms (throat becomes sore, watery eyes and etc). These are the signs of immune system fighting the hostile influenza. Once the invaders are destroyed, the antibodies will rapidly reduce in number. The symptoms gradually vanish.
However the ingenious mechanism of memory in our immune system exemplifies the beauty of nature. The remaining antibodies will remember how to attack if the specific influenza virus returns. Thus if second time a person is infected with the same influenza virus, the body will be quick to respond and kills off the virus rapidly.
Based on the previous posts you might compare this with the mechanism of how vaccines work.
Sources:
http://www.the-scientist.com/?articles.view/articleNo/29067/title/Building-Better-Proteins/ http://www.sanidadanimal.info/cursos/inmun/septimo3.htm http://www.dana.org/Publications/ReportDetails.aspx?id=44163
INFLUENZA 