Yes, interferon is a cytokine. It belongs to a broad class of signaling proteins that immune cells release to coordinate the body’s defense against threats. First discovered in 1957 by Alick Isaacs and Jean Lindenmann at Mill Hill in London, interferons got their name because they “interfere” with viral replication. Today they’re recognized as one of the most important cytokine families in human immunity, with roles that extend well beyond fighting viruses.
Where Interferons Fit in the Cytokine Family
Cytokines are a large group of small proteins that cells use to send signals to one another. Interleukins, tumor necrosis factor, and growth factors are all cytokines. Interferons are their own distinct subgroup within this family, defined by their shared ability to trigger antiviral defenses when they bind to specific receptors on cell surfaces.
What sets interferons apart from other cytokines like interleukins is their specialized antiviral machinery. When interferons dock onto a cell’s receptor, they activate an internal signaling cascade (called the JAK/STAT pathway) that switches on hundreds of genes collectively known as interferon-stimulated genes, or ISGs. These genes produce proteins that can block nearly every stage of a virus’s life cycle: entry into the cell, copying of its genetic material, assembly of new viral particles, and release from the cell. Other cytokines may recruit immune cells or trigger inflammation, but interferons are uniquely wired to put cells into a direct antiviral state.
Three Types of Interferon
Humans produce three classes of interferon, each with different members, sources, and roles.
Type I
This is the largest group, with 17 subtypes in humans, including 13 versions of interferon-alpha plus interferon-beta and a few others. Both immune cells and tissue-specific cells produce type I interferons, and virtually every cell in the body can respond to them through a shared receptor. They are the first-line antiviral signal: when a cell detects a virus, it releases type I interferons to warn neighboring cells and put them on high alert.
Type II
Type II interferon consists of a single protein, interferon-gamma. It is structurally unrelated to the other two classes. Natural killer cells and T cells are its primary producers, and it plays a broader immune-regulation role beyond antiviral defense. Interferon-gamma activates immune cells called macrophages, helping them kill bacteria and other pathogens that live inside cells. It’s also the molecule measured in tuberculosis blood tests (IGRAs), where a blood sample is exposed to TB-specific proteins and checked for interferon-gamma release as a sign of prior infection.
Type III
Type III interferons, also called interferon-lambda, have four known members in humans. Unlike type I interferons, which act on cells throughout the body, type III interferons mainly protect epithelial surfaces: the lining of the lungs, intestines, and liver. Their receptor is found almost exclusively on epithelial cells and a small subset of immune cells, making them a targeted first defense at the barriers where many infections begin.
How Interferons Stop Viruses
Once interferons bind to their receptor and activate the JAK/STAT signaling pathway inside a cell, that cell begins producing a wide arsenal of antiviral proteins. Each targets viruses at a different weak point.
- Blocking entry: A group of proteins called IFITMs can prevent viruses like influenza, dengue, Ebola, and SARS-CoV from entering cells, likely by disrupting the process that lets viral particles fuse with cell membranes.
- Destroying viral genetic material: Some proteins, like the OAS family, activate enzymes that chew up viral RNA. Others, like APOBEC3, introduce errors into the genomes of retroviruses like HIV during replication, rendering new copies defective.
- Stopping viral protein production: Proteins like PKR shut down the cell’s translation machinery so viruses can’t hijack it to build their own proteins. This is a scorched-earth tactic: the cell sacrifices some of its own function to starve the virus.
- Trapping new viruses: A protein called tetherin physically tethers newly made viral particles to the cell surface, preventing them from budding off and spreading to other cells. HIV-1 has evolved a specific countermeasure against tetherin, which hints at how effective this defense is.
With hundreds of interferon-stimulated genes, the system is highly redundant. Even if a virus evolves a way to dodge one defense, others are still active. Studies on hepatitis C found that when researchers tested eight different antiviral ISGs, each one independently impaired the virus’s ability to translate its genome after entering a cell, suggesting that ganging up on early replication steps is a core strategy.
When Interferons Turn Against the Body
Because interferons are such powerful immune activators, problems arise when the system doesn’t shut off properly. In autoimmune diseases, particularly systemic lupus erythematosus (SLE), researchers consistently find an “interferon signature,” an abnormally high expression of interferon-stimulated genes in patients’ blood cells.
The mechanism involves a self-amplifying loop. In lupus, circulating autoantibodies activate a type of white blood cell called neutrophils, which release tangled webs of DNA and antimicrobial proteins. These complexes then trigger specialized immune cells to produce massive amounts of interferon-alpha, up to 200 to 1,000 times more than normal. That flood of interferon further stimulates the immune system, which produces more autoantibodies, which activate more neutrophils, and the cycle continues. This feed-forward loop is a central driver of disease activity in lupus and is implicated in other autoimmune conditions including rheumatoid arthritis and systemic sclerosis.
The body does have a built-in brake system. Proteins called SOCS attach to the interferon receptor and its associated signaling molecules, blocking the pathway and halting the cascade. But in autoimmune disease, genetic variations in interferon pathway genes can tip the balance toward overproduction, lowering the threshold for both innate and adaptive immune responses.
Medical Uses of Synthetic Interferons
Because interferons are natural immune signals, pharmaceutical versions have been developed to treat several conditions. Interferon-beta injections are a long-established treatment for multiple sclerosis, where they help reduce the frequency of relapses. About two-thirds of patients on interferon-beta experience flu-like symptoms including fever, chills, headache, and muscle pain, typically starting a few hours after injection and lasting up to 24 hours. These side effects occur because the injected interferon triggers the release of other inflammatory cytokines, particularly interleukin-6, essentially mimicking a mild infection response.
Interferon-alpha therapies have been used for decades against chronic hepatitis C, though newer direct-acting antiviral drugs have largely replaced them for that purpose. In blood cancers called myeloproliferative neoplasms, interferons remain a frontline treatment. Ropeginterferon alfa-2b is the only FDA-approved interferon for polycythemia vera and is recommended as first-line therapy in the January 2025 NCCN guidelines. Pegylated interferon alfa-2a has also been widely used for these conditions, though recent supply shortages have pushed many patients toward newer formulations.
Interferon-gamma, meanwhile, found its niche not as a treatment but as a diagnostic tool. TB blood tests work by mixing a patient’s blood with proteins specific to the tuberculosis bacterium. If the person’s immune cells have previously encountered TB, their T cells will release interferon-gamma in response, producing a positive result. The CDC recommends these tests especially for people who have received the BCG vaccine, which can cause false positives on traditional skin tests. One limitation: it can take two to eight weeks after TB exposure for the immune system to mount a detectable interferon-gamma response, so recent contacts of TB patients should be retested 8 to 10 weeks after their last exposure.