Small secreted proteins produced by host cells in response to pathogens, particularly viruses, that modulate immune responses and confer resistance to infection in neighbouring cells.
Explanation
Interferons are a family of cytokines divided into three major types: type I (including interferon‑alpha and interferon‑beta), type II (interferon‑gamma) and type III (interferon‑lambda). Virus‑infected cells recognize viral nucleic acids through pattern‑recognition receptors and secrete type I interferons, which bind to interferon receptors on the surface of the same cell (autocrine) or nearby cells (paracrine). Signalling through the JAK‑STAT pathway induces hundreds of interferon‑stimulated genes that encode proteins capable of degrading viral RNA, inhibiting translation, enhancing antigen processing and up‑regulating cell surface major histocompatibility complex molecules. Type II interferon is produced mainly by activated T lymphocytes and natural killer cells and has potent immunomodulatory effects, activating macrophages and promoting a Th1‑biased immune response. Type III interferons act primarily on epithelial cells at mucosal surfaces. Together, interferons are key components of innate antiviral defence and bridge to adaptive immunity.
Examples and applications
During influenza or SARS‑CoV‑2 infection, airway epithelial cells and plasmacytoid dendritic cells release interferon‑alpha and interferon‑beta, which induce an antiviral state in the respiratory epithelium. Interferon‑gamma produced by CD4+ T helper 1 cells activates macrophages to kill intracellular pathogens such as Mycobacterium tuberculosis. Recombinant interferon‑alpha has been used to treat chronic hepatitis B and hepatitis C infections, although direct-acting antivirals have largely replaced it for hepatitis C. Interferon‑beta is used as a disease‑modifying therapy in multiple sclerosis to reduce relapse frequency. Genetic deficiencies in interferon signalling pathways can lead to severe susceptibility to viral diseases.
Interferons exemplify how cytokines orchestrate innate and adaptive immunity, and their therapeutic manipulation continues to inform the management of viral infections and immune disorders.
Related Terms: Cytokine, Innate immunity, Antiviral, NK cell, T cell