Cytokines and Chemokines: Signaling Molecules of Immunity
A splinter in your finger causes redness, swelling, and a flood of neutrophils within hours. A viral infection instead triggers fever, malaise, and antiviral resistance in cells far from the infection. Why the difference? Cells of the immune system cannot phone one another or send memos. They must communicate through diffusible molecules. These molecules broadcast a cell's status to its neighbors and to distant tissues. Cytokines and chemokines are that vocabulary. They are small secreted proteins. They convert a local sensing event into a coordinated, body-wide response. One example is a Toll-Like Receptors and Signaling-triggered detection of microbial molecules. This matters because immunity is fundamentally a problem of coordination. Trillions of cells must coordinate, and none can see the whole battlefield. A macrophage senses bacterial lipopolysaccharide at a wound site. It must recruit neutrophils from the blood. It must alert the liver to make acute-phase proteins. It must raise the hypothalamic set point to produce fever. Eventually it must summon lymphocytes for an adaptive response. It does all this without moving from its post. Cytokines carry these instructions as chemical messages. Chemokines are a specialized subset. They additionally encode directional information. They guide cells along concentration gradients to precise anatomical destinations. The big picture is that cytokines form an interconnected signaling network. This is not a simple lock-and-key system with one cytokine per effect. A single cytokine can act on many cell types with different consequences (pleiotropy). Several distinct cytokines can produce overlapping outcomes (redundancy). Cytokines also regulate each other's production and receptors in cascades and feedback loops. This network logic explains the extraordinary flexibility of immune responses. It also explains the danger of dysregulation. Examples range from the "cytokine storms" of severe sepsis and severe COVID-19. They also include the successes of cytokine-blocking drugs in autoimmune disease. This article builds the conceptual scaffold for understanding cytokine and chemokine classes. It covers their molecular mechanism of receptor engagement and cellular action. It covers the classic experiments that revealed how they work. It also covers the clinical and biotechnological uses that flow directly from this biology. It sets up later treatments of Inflammation: The Innate Response to Tissue Damage, The Acute-Phase Response, Type I Interferons and Antiviral Defense, and Natural Killer Cells and Missing-Self Recognition.
- Cytokines are small secreted proteins that let immune and other cells signal to each other; chemokines are a structurally distinct subfamily that direct directional cell migration.
- Major classes include interleukins, interferons, tumor necrosis factors, colony-stimulating factors, and chemokines (CXC, CC, CX3C, C).
- Cytokines act through specific cell-surface receptors, triggering signaling cascades (notably JAK-STAT) that change gene expression in the target cell.
- Cytokine action is pleiotropic, redundant, synergistic/antagonistic, and often organized in cascades — not one-cytokine-one-effect.
- Classic experiments — from Isaacs and Lindenmann's discovery of interferon to Baggiolini's identification of IL-8 — established these molecules as real, purified, gene-encoded mediators, not vague "factors."
- Cytokines are now major drug targets and drugs themselves: anti-TNF antibodies, IL-6 blockers, recombinant interferons, and CAR-T cytokine-release-syndrome management all exploit this biology directly.
Cytokines and chemokines are the molecular language that lets individual immune cells act as a coordinated system, translating local danger signals into precisely targeted, cell-type-specific instructions elsewhere in the body.
The core idea
A cytokine is a small (typically 5–20 kDa) secreted or membrane-bound protein that binds a specific receptor on a target cell to alter that cell's behavior — its gene expression, proliferation, differentiation, survival, or function. Cytokines are produced transiently and in small amounts by many cell types, in contrast to classical endocrine hormones, which are made by dedicated glands. A single cytokine can act in three modes: autocrine (on the cell that made it), paracrine (on nearby cells), or, at high concentrations during severe infection, endocrine (on distant organs, as when interleukin-6 reaches the liver to trigger the acute-phase response). The key functional properties that distinguish cytokine signaling from simple one-to-one messaging are pleiotropy (one cytokine, many target cells, many effects), redundancy (many cytokines, overlapping effect), synergy and antagonism between cytokines acting together, and cascade induction, where one cytokine induces production of others. Cytokines are grouped by structure and historical discovery into overlapping families: interleukins (originally "signals between leukocytes," now a numbered series IL-1 through beyond IL-40), interferons (IFN-α/β type I, IFN-γ type II, IFN-λ type III, named for their capacity to "interfere" with viral replication), tumor necrosis factors (TNF-α and lymphotoxin, potent inducers of inflammation and cell death pathways), and colony-stimulating factors (G-CSF, M-CSF, GM-CSF, which drive proliferation and differentiation of bone-marrow progenitors into mature leukocytes). Structurally, most cytokines fall into a small number of protein folds — the four-helix bundle (IL-2, IL-4, IL-6, GM-CSF, erythropoietin), the interleukin-1/fibroblast-growth-factor β-trefoil fold, and the TNF family's β-jellyroll trimer — reflecting shared evolutionary origins for large parts of the family despite very different names and functions.
Chemokines ("chemotactic cytokines") are a distinct structural subfamily, roughly 8–12 kDa, defined by a conserved pattern of cysteine residues that form disulfide bonds stabilizing a compact globular fold with a flexible N-terminus. They are classified by the spacing of their first two cysteines: CXC (one amino acid between the cysteines, e.g., CXCL8/IL-8), CC (adjacent cysteines, e.g., CCL2/MCP-1), CX3C (three amino acids between, the sole member fractalkine/CX3CL1), and C (only one cysteine, XCL1/lymphotactin). Unlike most cytokines, chemokines form immobilized concentration gradients — bound to glycosaminoglycans on endothelial surfaces and extracellular matrix — that leukocytes read directionally through G-protein-coupled receptors, converting a chemical gradient into a physical migration path. This gradient-reading property is what makes chemokines specialized address labels for cell trafficking, distinct from the broader instructional roles of other cytokines.
Mechanism — how cytokine and chemokine signaling actually works
Step 1 — Sensing and induction. An innate sensor, such as a Toll-like receptor on a macrophage or dendritic cell, detects a pathogen-associated molecular pattern (e.g., lipopolysaccharide via TLR4). This triggers intracellular signaling (MyD88/TRIF adaptors, NF-κB and IRF transcription factors) that activates transcription of cytokine genes — TNF-α, IL-1β, IL-6, and chemokines such as CXCL8 are among the earliest induced. Many cytokine genes, notably IL-1β, are synthesized as inactive pro-forms that require proteolytic cleavage (by caspase-1 within the inflammasome, for IL-1β) before secretion — an additional regulatory checkpoint that prevents indiscriminate release.
Step 2 — Secretion and local concentration gradients. Secreted cytokines diffuse from the producing cell. Because they are small and rapidly degraded or bound by receptors and matrix, their effective range is often only micrometers to millimeters — enough to signal within a lymph node or infected tissue but not usually across the whole body, except when produced in overwhelming quantity. Chemokines are actively retained on the luminal surface of blood vessel endothelium by binding to glycosaminoglycans (heparan sulfate), creating a stable, immobilized gradient rather than a freely diffusing cloud — essential for directing rolling leukocytes to exit the bloodstream at exactly the right spot.
Step 3 — Receptor engagement. Target cells display cytokine-specific receptors. Most cytokine receptors belong to a small number of superfamilies:
- Type I (hematopoietin) receptors — for IL-2 through IL-7, IL-9, IL-11 through IL-13, IL-15, IL-21, GM-CSF, and others — dimerize or form multi-chain complexes on ligand binding and signal through receptor-associated Janus kinases (JAKs).
- Type II receptors — for interferons and IL-10-family cytokines — also signal via JAK-STAT but with distinct receptor architecture.
- TNF receptor superfamily — trimeric receptors (TNFR1, TNFR2, Fas, CD40) that recruit adaptor proteins (TRADD, FADD) leading to NF-κB activation or, in some contexts, caspase-mediated apoptosis.
- IL-1 receptor family (including Toll-like receptors, sharing the cytoplasmic TIR domain) — signal through MyD88 to activate NF-κB and MAPK pathways.
- Chemokine receptors are seven-transmembrane G-protein-coupled receptors (GPCRs) coupled to Gαi proteins; ligand binding activates phospholipase C, generates a calcium flux, and activates small GTPases (Rac, Rho, Cdc42) that reorganize the actin cytoskeleton for directed movement.
Step 4 — Intracellular signal transduction. For the largest cytokine class, ligand-induced receptor dimerization brings together receptor-bound JAK kinases (JAK1, JAK2, JAK3, TYK2), which trans-phosphorylate each other and the receptor's cytoplasmic tail. This creates docking sites for STAT proteins (signal transducers and activators of transcription), which are themselves phosphorylated, dimerize, translocate to the nucleus, and bind DNA to activate specific gene programs. Different cytokines use different STAT combinations — IFN-γ predominantly activates STAT1, IL-4 activates STAT6, IL-6 activates STAT3 — which is a major reason why structurally related cytokines produce such different biological outcomes; specificity is encoded in the exact receptor-JAK-STAT combination engaged, not merely in "which cytokine is present." This signaling is actively terminated by SOCS (suppressor of cytokine signaling) proteins, providing negative feedback that limits response duration.
Step 5 — Directed cell migration (chemokines specifically). A neutrophil or lymphocyte in flowing blood first tethers and rolls along inflamed endothelium via selectins binding sialylated carbohydrates. Endothelium-displayed chemokines (e.g., CXCL8 presented on the luminal surface) engage GPCRs on the rolling leukocyte, triggering "inside-out" activation of integrins (LFA-1, VLA-4) from a low- to high-affinity conformation within seconds. High-affinity integrins bind endothelial adhesion molecules (ICAM-1, VCAM-1), arresting the cell firmly. The leukocyte then migrates along the chemokine gradient through the endothelial junction (diapedesis) and through the extracellular matrix toward the highest local chemokine concentration — typically the site of infection or damage, where resident macrophages continue producing the chemoattractant.
Step 6 — Feedback and resolution. As pathogen is cleared, cytokine production declines, anti-inflammatory cytokines (IL-10, TGF-β) dampen further activation, chemokine gradients dissipate, and recruited cells either die by apoptosis, are cleared by efferocytosis, or differentiate into memory populations — returning the tissue toward homeostasis.
The evidence — the classic experiments that established cytokine biology
Isaacs and Lindenmann, interferon (1957). Working at the National Institute for Medical Research in London, Alick Isaacs and Jean Lindenmann showed that chick chorioallantoic membranes exposed to heat-inactivated influenza virus produced a soluble factor that protected fresh membranes from live-virus infection. This factor acted not by neutralizing the virus directly, but by inducing a resistant antiviral state in the recipient cells — the first clear demonstration that a cell-produced protein could non-specifically confer antiviral protection on other cells before adaptive immunity had time to act. They named it interferon, since it "interfered" with viral replication. This experiment established, for the first time, that cells actively communicate a state of infection to their neighbors via secreted proteins — the founding observation of the entire cytokine field, and it directly foreshadows Type I Interferons and Antiviral Defense.
Dinarello and colleagues, endogenous pyrogen / IL-1 (1970s–1980s). Fevers had long been known to be induced by a substance released from leukocytes exposed to bacteria — "endogenous pyrogen" — distinguishing it from bacterial endotoxin itself (an "exogenous pyrogen"). Charles Dinarello purified this leukocyte-derived pyrogen to homogeneity and showed it was identical to what other laboratories, studying lymphocyte-activating factor, were calling interleukin-1. Purification and sequencing proved that a single molecular species could both activate lymphocytes and independently act directly on the hypothalamus to raise body temperature — an early, rigorous demonstration of cytokine pleiotropy (one molecule, multiple distinct physiological effects in different organs), foreshadowing The Acute-Phase Response.
Baggiolini and colleagues, discovery of IL-8/CXCL8 as the prototype chemokine (1987). Marco Baggiolini's group purified a small protein from stimulated human monocytes and showed it had potent, selective chemotactic activity for neutrophils specifically — unlike previously known chemoattractants such as complement fragment C5a or bacterial formyl peptides, which act on multiple leukocyte types. Sequencing revealed a novel structural motif (the CXC cysteine spacing), and its gene was cloned shortly after. This work established chemokines as a molecularly distinct family dedicated to directional, cell-type-specific recruitment, separate from the broader cytokine category, and set the classification scheme (CXC, CC, etc.) still used today.
Cytokine receptor cloning and JAK-STAT discovery (1988–1994). The molecular mechanism of cytokine action was nailed down when the IL-2 receptor and subsequently other cytokine receptors were cloned, revealing they lacked intrinsic kinase domains — unlike growth factor receptors such as EGFR. This was puzzling until James Darnell, Ian Kerr, and George Stark, studying interferon signaling, discovered that cytokine receptors instead recruit separate cytoplasmic tyrosine kinases (JAKs) and transcription factors that are directly activated by phosphorylation (STATs), rather than requiring the multi-step second-messenger cascades used by many other receptor systems. Genetic complementation experiments in interferon-unresponsive mutant cell lines — restoring responsiveness by transfecting individual JAK or STAT genes — proved each component's necessity, establishing JAK-STAT as a general, rapid, direct pathway from cell-surface receptor to nuclear gene transcription. This mechanistic clarity is why cytokine signaling can be pharmacologically targeted today with small-molecule JAK inhibitors.
Knockout mouse studies (1990s). Gene-targeted mice lacking specific cytokines or receptors provided definitive causal evidence for function in vivo, replacing correlation with genetics. IL-2 knockout mice, contrary to the prediction that IL-2 (originally called T-cell growth factor) would be essential for all T-cell proliferation, instead developed severe autoimmune disease and colitis, revealing an unexpected essential role for IL-2 in maintaining regulatory T cells — a striking lesson that a cytokine's name and first-discovered function can be quite different from its most physiologically essential role. TNF-α knockout mice showed defective granuloma formation and heightened susceptibility to Mycobacterium tuberculosis and Listeria, directly explaining why anti-TNF biologic drugs carry a boxed warning for tuberculosis reactivation in patients.
Worked examples
Example 1 — Reading a cytokine dose-response and receptor-blockade experiment. A researcher treats cultured monocytes with increasing concentrations of recombinant IL-6 and measures phosphorylated STAT3 (pSTAT3) by flow cytometry, then repeats the experiment in the presence of a saturating dose of tocilizumab (an anti-IL-6-receptor antibody used clinically in rheumatoid arthritis and cytokine release syndrome). Results:
| IL-6 (ng/mL) | % pSTAT3+ cells (no antibody) | % pSTAT3+ cells (+ tocilizumab) |
|---|---|---|
| 0 | 2% | 2% |
| 1 | 22% | 3% |
| 10 | 68% | 4% |
| 100 | 91% | 5% |
Reasoning: the dose-dependent rise in pSTAT3 with IL-6 alone confirms IL-6 signals through the JAK-STAT pathway in monocytes, consistent with IL-6 using a type I receptor complex (IL-6Rα plus the shared signal-transducer gp130) coupled to JAK1/JAK2 and STAT3. The near-complete abolition of the response by tocilizumab, even at the highest IL-6 dose, demonstrates that essentially all of this STAT3 activation requires IL-6 receptor engagement — the antibody is competitively blocking ligand binding rather than acting downstream. This is precisely the pharmacologic logic behind using IL-6-receptor blockade to treat cytokine-driven diseases: because tocilizumab acts at the receptor rather than by neutralizing free IL-6 molecule-for-molecule, it remains effective even when IL-6 concentrations rise sharply, as they do in severe inflammation or CAR-T-cell-associated cytokine release syndrome.
Example 2 — A clinical scenario integrating chemokine gradients and receptor genetics. A 24-year-old man is found to carry a homozygous 32-base-pair deletion in the CCR5 gene (CCR5-Δ32/Δ32), which prevents surface expression of the chemokine receptor CCR5. He is otherwise healthy, has a normal history of bacterial and viral infections, but has been repeatedly exposed to HIV-1 through a partner without becoming infected. Reasoning: CCR5 is a chemokine receptor for CC-chemokines (CCL3/MIP-1α, CCL4/MIP-1β, CCL5/RANTES) normally used to recruit macrophages and T cells to sites of inflammation — but it is also the principal co-receptor that R5-tropic strains of HIV-1 use, together with CD4, to enter target T cells and macrophages. Because CCR5-Δ32/Δ32 individuals lack functional CCR5 on the cell surface, R5-tropic HIV cannot efficiently enter their CD4+ T cells, conferring near-complete resistance to infection by these strains (though not to the less common CXCR4-using X4-tropic strains, which use a different chemokine receptor). This natural human "knockout" mirrors the mouse knockout logic used experimentally: because CCR5 is largely redundant with other chemokine receptors for ordinary immune surveillance, its absence causes little baseline immune deficit, while its co-option by a pathogen explains why blocking it (as the drug maraviroc does) is a viable and safe HIV therapy — directly illustrating how chemokine receptor biology can be exploited by pathogens and by pharmacology alike.
Common misconceptions & traps
| Misconception | Why it's wrong |
|---|---|
| "Each cytokine has one specific job." | Pleiotropy means most cytokines act on many cell types with different, sometimes opposite, effects depending on context and receptor expression. |
| "Cytokines are hormones, just smaller." | Cytokines are typically produced locally and transiently by many cell types, not by dedicated endocrine glands, and usually act over short range. |
| "More cytokine always means a stronger, better immune response." | Excess cytokine production (cytokine storm/release syndrome) causes tissue damage, shock, and death — dose and timing matter enormously. |
| "Chemokines are just weaker versions of other cytokines." | Chemokines are a structurally distinct GPCR-signaling family specialized for directional cell migration, not a lesser cytokine subtype. |
| "IL-2 exists mainly to make T cells proliferate, since that's its historic name." | Knockout studies show IL-2's most essential in vivo role is maintaining regulatory T cells and preventing autoimmunity, not driving effector proliferation. |
| "Interferon 'interferes' by directly killing the virus." | Interferon acts indirectly, inducing an antiviral gene program (e.g., PKR, OAS) in bystander cells rather than attacking virions itself. |
The scientist behind it
Charles A. Dinarello has spent much of his career pursuing a single molecular question with unusual persistence: what actually is "endogenous pyrogen," the mysterious fever-inducing substance released by white blood cells during infection? Trained as a physician-scientist, Dinarello worked for years at the U.S. National Institutes of Health and later the University of Colorado, using painstaking classical biochemical purification — a technically brutal task given the vanishingly small quantities of active protein present in leukocyte-conditioned medium — to isolate and finally sequence this pyrogen in the early 1980s. His work proved it was the same molecule several other laboratories, studying T-cell activation, had been calling "lymphocyte-activating factor," unifying these separate research threads under the name interleukin-1. This single act of molecular identification was pivotal: it converted a vague physiological description ("something that causes fever") into a defined, clonable gene and protein that could be studied mechanistically, produced recombinantly, and eventually targeted therapeutically (IL-1-blocking drugs such as anakinra are now used to treat autoinflammatory diseases and severe gout flares). Dinarello's career illustrates a recurring pattern in cytokine biology: a phenomenon is observed physiologically long before its molecular cause is known, and rigorous purification-to-homogeneity is what turns folklore into a mechanism.
Origins · 1 minCharles A. Dinarello and how this was discovered
Where this matters today
Cytokine and chemokine biology is now central to clinical medicine, not just basic immunology. Anti-TNF biologics (infliximab, adalimumab, etanercept) treat rheumatoid arthritis, inflammatory bowel disease, and psoriasis by neutralizing TNF-α, directly built on the knockout-mouse evidence that TNF drives chronic inflammatory tissue damage. IL-6-receptor blockade (tocilizumab) treats rheumatoid arthritis and, notably, was repurposed during the COVID-19 pandemic to dampen the cytokine storm of severe respiratory failure, and remains a first-line treatment for cytokine release syndrome after CAR-T-cell therapy. Recombinant interferons (IFN-α, IFN-β) are used against chronic viral hepatitis and multiple sclerosis respectively. Recombinant colony-stimulating factors (G-CSF/filgrastim) are given to cancer patients after chemotherapy to accelerate neutrophil recovery and prevent life-threatening neutropenic infection, a direct clinical application of cytokine-driven hematopoiesis. Chemokine receptor antagonism underlies maraviroc, a CCR5 blocker used in HIV treatment, exploiting the same receptor biology revealed by the natural CCR5-Δ32 human knockout. In agriculture and veterinary medicine, recombinant cytokines and cytokine-adjuvant formulations improve vaccine responses in livestock. In cancer immunotherapy, engineered cytokines (IL-2 variants, IL-15 superagonists) are being redesigned to selectively expand tumor-killing lymphocytes while minimizing the severe toxicity of native high-dose IL-2 therapy — a direct engineering application of the receptor-selectivity principles this article describes. Understanding cytokine networks is equally essential background for the mechanisms of Inflammation: The Innate Response to Tissue Damage and Natural Killer Cells and Missing-Self Recognition, both of which depend on precisely the signaling logic outlined here.
Ask the tutor: How do IL-4, IL-13, and IL-10 skew a T-helper response toward Th2 versus regulatory outcomes, and why does this matter for allergy and autoimmunity?
Are cytokines and hormones fundamentally different?
They differ mainly in source, range, and regulation rather than chemical nature. Hormones (like insulin or thyroxine) are produced by dedicated endocrine glands, travel systemically through the bloodstream, and act at low, tightly regulated concentrations over long distances. Cytokines are produced transiently by many different cell types (immune cells, epithelium, fibroblasts) in response to a stimulus, usually act locally in autocrine or paracrine fashion, and are typically undetectable at baseline, rising sharply only during activation. During severe systemic infection, cytokines like IL-6 can reach the bloodstream in hormone-like concentrations and act on distant organs (the liver, the hypothalamus), blurring the distinction — but their default mode of action is local and cell-triggered rather than glandular and continuous.
Why do some cytokines cause fever while others cause redness and swelling — aren't they all just "inflammation signals"?
Different cytokines engage different receptors on different target tissues, producing distinct downstream programs even though they are co-released during the same infection. IL-1 and IL-6 reach the hypothalamus through the bloodstream and, acting on receptors there, raise the thermoregulatory set point via prostaglandin E2 production, causing fever. TNF-α and IL-1 act locally on vascular endothelium to increase blood flow and vascular permeability (redness, swelling) and to upregulate adhesion molecules that recruit neutrophils. The same infection triggers all of these cytokines together, but each exerts its effect only where its receptor is expressed and functional — fever is a hypothalamic response, swelling is a local vascular response, and both stem from overlapping but non-identical cytokine signals.
What exactly is a "cytokine storm," and why is it dangerous?
A cytokine storm is a severe, self-amplifying overproduction of pro-inflammatory cytokines (TNF-α, IL-6, IL-1, interferons) that outpaces the body's normal regulatory checkpoints. Instead of a proportionate, localized response, cytokines reach systemic, near-toxic concentrations, causing widespread vascular leak, hypotension, disseminated intravascular coagulation, and multi-organ failure. It occurs in severe sepsis, some severe viral infections (including severe COVID-19), and as a complication of CAR-T-cell cancer therapy (cytokine release syndrome), where engineered T cells kill tumor cells so effectively that surrounding immune cells are massively activated. The danger illustrates that cytokine networks have amplifying feedback loops; clinical management often specifically targets IL-6 (tocilizumab) or TNF to break the amplification cycle rather than suppressing the whole immune system.
How does a chemokine gradient actually guide a cell, given that diffusion should quickly even it out?
Chemokines are not left to diffuse freely; they are captured and immobilized on glycosaminoglycan chains (heparan sulfate) displayed on endothelial cell surfaces and in extracellular matrix. This anchoring creates a stable, spatially fixed gradient that a migrating leukocyte can sense using surface chemokine receptors distributed around its membrane. The cell compares receptor occupancy on its leading versus trailing edge, biasing actin polymerization toward the side with higher chemokine engagement, producing directed crawling (chemotaxis) rather than random wandering. This immobilization is essential; a freely diffusing signal would equilibrate within minutes and could not reliably direct a cell over the multi-hour timescale of an inflammatory response.
Why does blocking TNF-α treat rheumatoid arthritis but sometimes cause tuberculosis to reactivate?
TNF-α has dual, context-dependent roles: it is a central driver of the chronic synovial inflammation that damages joints in rheumatoid arthritis, but it is also required to maintain granulomas — organized immune structures that wall off latent Mycobacterium tuberculosis bacteria and prevent their reactivation. TNF-α knockout mice were shown experimentally to fail at granuloma maintenance and succumb rapidly to mycobacterial and Listeria infection. Anti-TNF biologic drugs (infliximab, adalimumab) block this same pathway systemically, which is therapeutically desirable in the joint but can allow dormant TB bacteria, previously contained by TNF-dependent granulomas, to reactivate — which is why patients are screened for latent TB before starting anti-TNF therapy.
Is it accurate to say interferons directly kill viruses, the way antibodies neutralize them?
No — this is a common but inaccurate simplification. Interferons act indirectly: type I interferons (IFN-α/β) secreted by an infected cell bind interferon receptors on neighboring, still-uninfected cells and activate JAK-STAT signaling (chiefly via STAT1/STAT2), inducing transcription of hundreds of interferon-stimulated genes. These genes encode antiviral effector proteins — such as protein kinase R, which shuts down protein synthesis in a cell if double-stranded viral RNA is detected, and the OAS/RNase L system, which degrades viral RNA — that make the recipient cell resistant to establishing infection. Interferon does not bind or destroy virus particles directly; it reprograms cellular gene expression to pre-emptively resist infection, a mechanism developed further in Type I Interferons and Antiviral Defense.
Why are chemokine receptors, which are GPCRs, considered part of "innate" trafficking rather than adaptive immune specificity?
Chemokine receptors direct which tissue a leukocyte migrates to, but they don't confer antigen specificity — that is the job of the T-cell receptor or B-cell receptor in adaptive immunity, layered on top of chemokine-guided positioning. A naive T cell, a memory T cell, and a neutrophil each express distinct combinations of chemokine receptors (e.g., CCR7 directs naive T cells to lymph nodes, CXCR3 directs activated T cells to inflamed tissue), so chemokine signaling determines cellular geography and timing of immune responses across both innate and adaptive arms. This is why chemokine receptor expression patterns are used clinically and experimentally as markers to distinguish naive, effector, and memory lymphocyte subsets, even though the receptors themselves have no role in recognizing specific antigens.