When cells are infected with viruses, the innate immune response activates production of Type I Interferon (interferon-alpha and interferon-beta). Since the interferon response is part of the innate, not the adaptive, immune system, the response is general and not specific to a particular virus.
The production of interferon leads to several important outcomes. First, interferon disrupts the virus' ability to replicate within the cell. Second, interferon binds to natural killer (NK) cell interferon receptors, causing activation of the NK cells. Third, interferon leads to upregulation of ligands on the infected cell's surface that are specific for NK cells, which allows NK cells to interact with and kill virus-infected cells. Lastly, interferon secretion by the infected cell has a paracrine function: secreted interferon can bind to uninfected neighboring cells and help these cells avoid viral infection.
Biron et al. discuss the role of NK cells in mediating significant levels of cytotoxic activity in viral defense. People without NK cells have increased susceptibility to viral infections, since this early mode of viral clearance from the body is lacking. Instead, these patients must wait until cytotoxic T cells of the adaptive immune system are ready to attack the infected cells. Often, these patients need antiviral medication to better fight the virus.
Takaoka and Yanai describe the signaling pathway leading to interferon production. Pattern recognition receptors (proteins expressed by cells of the innate immune system to identify pathogen-associated molecular patterns) are activated by pathogenic antigens, and this causes the downstream activation of interferon production which interfere with viral replication in host cells.
Illustration A depicts some of the important steps in the interferon response.
Illustration B shows the major downstream signaling pathways in the interferon response.
Answers 1, 3-5: These choices are incorrect since they can all be induced by interferon.
Biron CA, Nguyen KB, Pien GC, Cousens LP, Salazar-Mather TP. Natural killer cells in antiviral defense: function and regulation by innate cytokines. Annu Rev Immunol. 1999;17
PMID:10358757 (Link to Abstract)
Takaoka A, Yanai H. Interferon signalling network in innate defence. Cell Microbiol. 2006 Jun;8(6)
PMID:16681834 (Link to Abstract)