Influenza infection and immunological memory responses

Influenza virus is one of the most important respiratory pathogens, which leads to a high degree of morbidity and mortality in humans every year. Despite the severity of influenza, the controlling the virus still remains to be elucidated due to their well-developed escaping mechanisms such as antigenic drifts and shifts. Needless to say, these active variations are hindrances to successful treatment or prevention, and to the development of vaccine(s). Currently, three different strains are annually recommended by World Health Organization (WHO) for the vaccines to be commercialized against influenza virus. Although they appear to induce a reasonable degree of protection, it is essential that we understand thoroughly how the immunological memory responses against influenza, which is fundamental basis for the development of successful vaccine, are generated and contribute to the host immunity. In case of new pandemic influenza virus (H1N1, swine-origin influenza virus, SOIV), generally older are not recommended for the early vaccination owing to the low incidence of the infection with SOIV. Many researchers pointed that older might be exposed to influenza viruses that are similar to current pandemic virus during their youth and such experiences would allow them to have antibodies that partially protect SOIV infection. Moreover, the memory responses play an important role in cross protection by inducing heterosubtypic immunity against different subtype of influenza virus. In addition, the comprehension of memory responses against influenza is a fundamental step in order to germinate novel type of vaccine. Due to intense labor of annual vaccination, the researchers pursue the development of universal vaccine using relatively conserved virus protein. The extracellular domain of M2 virus protein (M2e) is potential target for this "One for all" influenza vaccine. However, the memory responses against influenza are not fully understood. In this review, we focus on the immunological memory responses against influenza virus. Not only B cell memory that is pivotal to generate antigen-specific antibodies but also T cell memory which would play an essential role against re-infection of influenza virus is dealt in this review. Additionally, we discuss how the understanding of immunological memory response can contribute to the vaccine development and the strategy to manage the infection with influenza virus.