section_path: Mechanism/Immunology
section_name: Immunology
paragraph_text: Immunology
  
  Cells possess sensors to detect viral RNA, which can then induce interferon production. Interferons mediate expression of antiviral proteins and proteins that recruit immune cells to the infection site, and they notify nearby uninfected cells of infection. Some infected cells release pro-inflammatory cytokines that recruit immune cells to the site of infection. Immune cells control viral infection by killing infected cells and phagocytizing viral particles and apoptotic cells. An exacerbated immune response can harm the host organism through a cytokine storm. To counter the immune response, influenza viruses encode various non-structural proteins, including NS1, NEP, PB1-F2, and PA-X, that are involved in curtailing the host immune response by suppressing interferon production and host gene expression.
  
  B cells, a type of white blood cell, produce antibodies that bind to influenza antigens HA and NA (or HEF) and other proteins to a lesser degree. Once bound to these proteins, antibodies block virions from binding to cellular receptors, neutralizing the virus. In humans, a sizeable antibody response occurs about one week after viral exposure. This antibody response is typically robust and long-lasting, especially for influenza C virus and influenza D virus. People exposed to a certain strain in childhood still possess antibodies to that strain at a reasonable level later in life, which can provide some protection to related strains. There is, however, an " original antigenic sin ", in which the first HA subtype a person is exposed to influences the antibody-based immune response to future infections and vaccines.