The first line of defence corresponds to innate immunity, which is able to act immediately after an aggression  In the case of influenza infection, the chemical mediator alpha interferon (IFN-α) plays a key role, creating an “antiviral” state in the cells, making them more difficult to infect. In addition, pro-inflammatory cytokines are responsible for inflammation in the respiratory tract and the onset of the typical symptoms of the disease. Finally, the cells of the innate immune response responsible for the destruction of infected cells are the natural killer cells. However, despite the beneficial effects of the innate immune response in the first moments after infection, effective control of the infection occurs with the development of the adaptive immune response, which develops during the course of the infection and is specific to the agent causing the infection. This type of immunity is also responsible for the prevention of reinfections. Hence, the degree of protection of an animal is dependent upon the similarity between the virus that caused the first infection, or was the basis of a vaccine, and the virus that produces a second infection.