What type of immune response is initiated when a person becomes infected with viral flu? b cells activate t cells. t cells merge with b cells. b cells produce antibodies specific to the flu virus. t cells produce antibodies common to all pathogens.

When someone is infected with a sickness usually your white blood cells and antibodies come to help. Bacteria is easier to shoo away while viral infections are harder to get rid of and need different medication to get rid of. 
The vaccines doctors give you for the flu are supposed to get your body used to the disease and help your body become immune. It trains your cells to fight the disease. 
If you've been vaccinated your B cells should produce antibodies for the flu virus since your body is able to identify that it is the Flu, if you haven't been vaccinated then your T cells will produce an antibody that is used for all other pathogens to see what works against the flu. (sorry for taking so long)

B cells produce antibodies specific to the flu

We can plant more trees and plants.