What Are Inflammatory Mediators and Their Function?

Inflammatory mediators are signaling molecules your body releases in response to injury, infection, or tissue damage, and they orchestrate nearly every aspect of the inflammatory response: widening blood vessels, making them leaky, recruiting immune cells, generating pain, and raising body temperature. They range from small molecules like histamine that act in seconds to proteins like cytokines that sustain inflammation over days or weeks. Understanding these mediators matters because they sit at the crossroads of healing and disease. When they work correctly, they help you recover from a cut or fight off an invading pathogen. When they spiral out of control, they drive conditions from arthritis to sepsis.

The Two Big Families

Inflammatory mediators fall broadly into two camps based on where they come from. Cell-derived mediators are produced and released by immune cells, endothelial cells, or damaged tissue at the site of injury. These include histamine, serotonin, prostaglandins, leukotrienes, cytokines, and chemokines. Plasma-derived mediators, by contrast, circulate in the blood in inactive forms and get switched on by enzymatic cascades when inflammation starts. The complement system, the kinin system, and the clotting cascade all generate plasma-derived mediators. In practice, both families work together simultaneously, amplifying each other’s signals.

Histamine, Serotonin, and the Earliest Response

Within seconds of tissue injury, mast cells near the damaged area dump their stored histamine into the surrounding tissue. Histamine immediately causes nearby blood vessels to dilate and become more permeable, which is why an insect bite swells and reddens almost instantly. Serotonin, better known for its role in the brain, does similar work in the periphery: it leaks from activated platelets and contributes to vascular permeability. Both histamine and serotonin also appear to have roles beyond simple inflammation. Research has shown that, like vascular endothelial growth factor (VEGF-A), histamine and serotonin can trigger new blood vessel formation on a time scale consistent with acute inflammation, partly by activating a transcription factor called Nur77 in endothelial cells.¹ That dual role makes sense: injured tissue needs new blood supply to heal, and the same molecules that kick off the alarm also start laying the groundwork for repair.

Prostaglandins, Thromboxanes, and Leukotrienes

When cell membranes are disrupted by injury, an enzyme called phospholipase A2 frees arachidonic acid from the membrane. What happens next depends on which enzyme gets to it first. The enzyme cyclo-oxygenase (COX) converts arachidonic acid into prostaglandins, prostacyclin, and thromboxanes, while a separate enzyme, 5-lipoxygenase, produces leukotrienes.² This branching pathway matters because it explains why different drugs target different symptoms. Aspirin and ibuprofen block COX, reducing prostaglandin production and thereby easing pain, swelling, and fever. Asthma medications like montelukast block leukotriene receptors instead, because leukotrienes are especially potent at constricting airways and attracting immune cells into the lungs.

Prostaglandins are among the most versatile mediators. Prostaglandin E2, for example, sensitizes nerve endings to pain, promotes vasodilation, and plays the central role in fever. It does this by binding to receptors in a region of the hypothalamus called the preoptic area, which then resets the body’s thermostat upward.³ Animal studies suggest that more than one receptor subtype in this brain region contributes to the fever response.⁴ That is why fever is such a reliable symptom of inflammation: the prostaglandin pathway feeding into it is robust and multi-layered.

Cytokines and Chemokines

If histamine and prostaglandins are the immediate alarm bells, cytokines are the sustained broadcast. Cytokines are small proteins released mainly by immune cells, and they coordinate nearly every phase of the inflammatory process. Two of the most studied pro-inflammatory cytokines are tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6). Under normal conditions, IL-6 engages receptors on immune cells and promotes further cytokine production, phagocytosis, and cell migration.⁵ TNF-α, for its part, has far-reaching metabolic effects. In obesity, elevated TNF-α in muscle and fat tissue can interfere with insulin signaling, contributing to insulin resistance.⁶ That link between chronic low-grade inflammation and metabolic disease is one reason inflammatory mediators have become central to research on diabetes and cardiovascular risk.

Chemokines are a specialized subset of cytokines whose main job is directing immune-cell traffic. They work as chemical breadcrumb trails, guiding white blood cells from the bloodstream to exactly where they are needed in the tissue.⁷ Without chemokines, immune cells would circulate aimlessly; with them, they home in on damaged or infected tissue with surprising precision.

The Complement System and Bradykinin

The complement system is a cascade of proteins in the blood that activates when it detects foreign surfaces like bacterial cell walls. Among its products are small fragments called anaphylatoxins, particularly C3a and C5a. These fragments are powerful inflammatory signals: they attract and activate immune cells such as granulocytes, mast cells, and macrophages.⁸ C3a and C5a bind to specific receptors at very low concentrations, meaning even trace amounts can produce a strong response.⁹ The “anaphylatoxin” label hints at their potency: in an uncontrolled setting, they can trigger anaphylaxis-like reactions.

Bradykinin, generated by the kinin system, is another plasma-derived mediator with a significant role. It causes blood vessels to dilate and become leaky, and it helps regulate blood pressure under normal conditions.¹⁰ During inflammation, bradykinin is one of the most important drivers of fluid leakage into tissue (the swelling you feel around a wound) and also stimulates pain receptors in collaboration with prostaglandins and other co-factors.¹¹ If you have ever wondered why a bee sting both swells and hurts so intensely, bradykinin and prostaglandins working together are a big part of the answer.

How Inflammatory Mediators Cause Pain

Pain during inflammation is not a single mediator’s doing. It happens because a cocktail of mediators converges on sensory nerve endings (nociceptors), lowering their firing threshold so that stimuli which would normally feel mild become painful. This phenomenon is called peripheral sensitization, and a wide range of mediators contribute: bradykinin, prostaglandins, serotonin, histamine, ATP, and nerve growth factor (NGF) all play a part.¹² Many of these mediators converge on a heat-sensing channel in nerve endings called TRPV1. Normally TRPV1 responds to high temperatures and capsaicin (the compound in chili peppers). When inflammatory mediators sensitize TRPV1, it fires at lower temperatures, which is why an inflamed area feels hot and painful to the touch even at normal body temperature.

Sensory nerves themselves then add to the inflammatory mix. When a nerve ending is activated, it can trigger an “axon reflex” that releases neuropeptides like substance P and CGRP from neighboring branches of the same nerve. These neuropeptides dilate blood vessels and increase permeability, expanding the zone of redness and swelling around the original injury.¹³ This neurogenic inflammation explains why a minor scrape can produce a surprisingly large flare of redness. The nerve endings are actively propagating the inflammatory signal outward. Substance P and CGRP also appear to play a beneficial role in wound healing later on, influencing the recovery of healthy blood flow in damaged tissue.¹⁴

How the Body Starts the Alarm

For the whole mediator cascade to start, the body first has to detect that something is wrong. Immune cells carry pattern recognition receptors that sense molecular signatures associated with invaders or damage. Toll-like receptors (TLRs) are the best known of these sensors. TLR3, for example, detects double-stranded RNA, the genetic material of many viruses. Other sensors, including RIG-I-like receptors and the NLRP3 inflammasome complex, provide backup detection of the same types of signals.¹⁵ Once these receptors are tripped, the cell ramps up production of cytokines, prostaglandins, and reactive oxygen species, feeding directly into the mediator cascades described above. The redundancy is deliberate: having multiple sensors for the same threat means a pathogen cannot easily slip past the alarm by disabling one detector.

Reactive oxygen and nitrogen species produced during this alarm phase are mediators in their own right. Nitric oxide (NO), generated by inducible nitric oxide synthase (iNOS), helps kill bacteria and dilates blood vessels. But its production is tightly linked to oxidative stress. High levels of reactive oxygen species can actually impair iNOS function, shifting it from producing protective NO to generating superoxide instead, which can damage the body’s own tissues.¹⁶ That switch from helpful to harmful is a recurring theme with inflammatory mediators: the same molecules that protect you in measured amounts can injure you in excess.

Turning Inflammation Off

Inflammation is not meant to run indefinitely. The body produces a separate class of mediators whose job is to actively shut the process down and restore normal tissue function. These are called specialized pro-resolving mediators (SPMs), and they include resolvins, protectins, and maresins. Many of them are derived from omega-3 fatty acids, particularly DHA (docosahexaenoic acid).¹⁷ SPMs work by limiting further immune cell recruitment, encouraging immune cells to clean up debris from the battle (a process called efferocytosis), and promoting tissue repair.¹⁸

The resolution phase is not simply the absence of pro-inflammatory signals. It is an active, coordinated process with its own mediator toolkit. When resolution fails, chronic inflammation results. This distinction matters because many chronic inflammatory diseases, from rheumatoid arthritis to atherosclerosis, may not be caused so much by too many pro-inflammatory mediators as by too few pro-resolving ones. The research on SPMs has opened up a new therapeutic strategy: rather than just blocking inflammation, you might be able to treat disease by boosting the body’s resolution machinery.

When Mediators Go Into Overdrive

The most dramatic example of inflammatory mediators doing harm is the cytokine storm: a runaway, self-amplifying release of pro-inflammatory cytokines that can cause multiple organ failure. Sepsis is the classic clinical scenario. It is a life-threatening condition marked by uncontrolled systemic inflammation, excessive cytokine release, inflammatory cell death, and organ dysfunction.¹⁹ The immune system essentially overreacts to infection, and the flood of mediators damages the body’s own tissues more than the pathogen does. Severe COVID-19 brought the concept of cytokine storms into public awareness, but the phenomenon has been recognized in sepsis, certain autoimmune flares, and some cancer immunotherapies for much longer.

Measuring Inflammation in the Clinic

Doctors do not typically measure individual mediators like histamine or IL-6 in routine clinical practice. Instead, they rely on downstream markers that reflect overall inflammatory activity. The two most common are C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR). CRP is a protein the liver produces in response to IL-6 and other cytokines, and it rises and falls relatively quickly with changes in inflammation. ESR measures how fast red blood cells settle in a tube over an hour, which speeds up when inflammatory proteins in the blood cause red cells to clump. CRP is generally considered the better indicator: it responds faster and is less affected by confounding factors like age, sex, and kidney function.²⁰

That said, neither marker tells you what is causing the inflammation. Elevated CRP could reflect an infection, a flare of inflammatory bowel disease, a heart attack, or a tumor. CRP and ESR thresholds can help gauge disease severity when the diagnosis is already known. In pulmonary tuberculosis, for instance, both markers correlate with how advanced the infection is.²¹ In inflammatory joint disease, specific CRP and ESR thresholds can help identify patients with very high disease activity, though both markers have relatively low sensitivity overall, meaning a normal result does not rule inflammation out.²²

Drugs That Target Inflammatory Mediators

Most anti-inflammatory drugs work by interfering with one or more of these mediator pathways. Non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen block the COX enzymes, cutting off prostaglandin and thromboxane production. Antihistamines block histamine receptors, mainly helping with allergic reactions and itching. Leukotriene-receptor antagonists target the other branch of the arachidonic acid pathway and are used primarily in asthma.

Corticosteroids, such as prednisone and dexamethasone, take a broader approach. They enter cells, bind to a receptor in the cytoplasm, and the resulting complex moves into the nucleus, where it suppresses the activity of transcription factors that drive production of many inflammatory genes at once. The upshot is reduced output of cytokines, enzymes, receptors, and adhesion molecules across the board.²³ That breadth is what makes corticosteroids both powerful and prone to side effects: they do not just suppress the problematic mediators, they dampen much of the immune response.

Biologic therapies represent the most precise approach. These are engineered antibodies or receptor-blocking proteins designed to neutralize a single mediator or receptor. TNF-α inhibitors, for instance, have transformed the treatment of rheumatoid arthritis and other autoimmune diseases, and anti-IL-6 receptor drugs like tocilizumab target the IL-6 pathway.²⁴ For patients who do not respond to conventional immunosuppression, these targeted biologics can make a meaningful difference in diseases as varied as inflammatory eye disease and systemic lupus.²⁵

Gut Bacteria and Inflammatory Tone

Your gut microbiome has a surprisingly large influence on the balance of inflammatory mediators throughout your body. Gut bacteria ferment dietary fiber into short-chain fatty acids (SCFAs), primarily acetate, propionate, and butyrate.²⁶ These SCFAs help maintain the gut lining, regulate immune cell behavior, and tamp down excessive inflammation. When SCFA-producing bacteria are depleted, whether through poor diet, antibiotics, or illness, the resulting deficiency has been linked to the development of inflammatory bowel diseases, colorectal cancer, and metabolic disorders.²⁷ In this way, the mediators that keep inflammation in check are not all made by your own cells; some depend on the microbial community living in your colon.

Inflammaging and Chronic Low-Grade Inflammation

As people age, the balance of inflammatory mediators shifts in a troubling direction. Older adults tend to have modestly elevated circulating levels of cytokines like IL-6, TNF-α, and CRP even in the absence of any acute illness. This state has been dubbed “inflammaging,” and it appears to accelerate many of the diseases associated with growing old, from cardiovascular disease to neurodegeneration.²⁸

A key driver of inflammaging is the accumulation of senescent cells, which are cells that have stopped dividing but refuse to die. These cells adopt what researchers call a senescence-associated secretory phenotype, or SASP, meaning they pump out pro-inflammatory cytokines, chemokines, growth factors, and protein-degrading enzymes. Multiple SASP-related mediators are dysregulated in age-related inflammatory diseases like psoriasis.²⁹ Experimental drugs called senolytics, designed to clear senescent cells, represent one emerging strategy to address the problem at its source rather than merely blocking individual mediators downstream.

Inflammatory Mediators Across the Animal Kingdom

Humans did not invent inflammation. The basic mediator toolkit is ancient and shared across an enormous range of species. Invertebrates and fish possess eicosanoid pathways (the arachidonic acid derivatives that produce prostaglandins and leukotrienes), cytokines, and adhesion molecules that serve essentially the same function as ours: detecting damage, recruiting defense cells, and sealing off threatened tissue.³⁰ Inflammation is apparently such an effective first-line defense that evolution has preserved its core chemistry for hundreds of millions of years. The conservation of these pathways is also why animal models can teach us so much about human inflammatory disease, though the details differ enough between species that drug candidates still sometimes fail when they move from the lab bench to human trials.