When a person is exposed to an influenza virus, the innate and subsequently the adaptive or specific immune response kicks in and provides defence against the invading virus. Depending on the nature of the virus and the effectiveness of the immune response, the infected individual suffers from more or less severe consequences of the influenza virus infection: in the process of developing specific immunity, the body produces a specific response, consisting of specific T cells and specific antibodies that fight off the infection. Once the immune system has been exposed to an invading virus, it stores this information in the form of educated memory cells, resulting in an immunological memory that upon re-exposure to the same or closely related virus reacts much faster to produce the specific T cells and antibodies than the non-primed immune system.
Influenza vaccines against seasonal influenza take advantage of this adaptive immune system by exposing the body before the seasonal influenza epidemic starts, through vaccination with viruses in an inactivated or weakened form or their relevant components. Thus, instead of suffering the natural virus infection and risking its consequences, vaccines induce immune and memory responses similar to those of the natural virus infections. This results in immunity towards the influenza viruses that are expected to circulate in the following seasons.