What Is Inflammation and Why It Matters for Almost Every Disease

Redness, heat, swelling, and pain. The Roman encyclopedist Aulus Cornelius Celsus listed these four signs of inflammation roughly 2,000 years ago, in Latin as rubor, calor, tumor, and dolor. A fifth, loss of function (functio laesa), was added later. Inflammation is the body's coordinated response to injury or infection, and it is essential for survival. It also does damage when it persists, and researchers now connect low-grade, long-running inflammation to heart disease, type 2 diabetes, some cancers, Alzheimer's disease, and depression.

Educational content, not medical advice. Talk to your doctor before changing any medication, treatment, or routine. Information here reflects current research but every person's situation differs.

Key takeaways

• Acute inflammation is a precisely coordinated protective response. It begins within minutes and typically resolves within days, and resolution is an active process rather than a fade-out.
• Chronic low-grade inflammation is a persistent, lower-level immune activation. When it appears with aging it is called "inflammaging," a term Claudio Franceschi introduced in 2000.
• Key mediators include the cytokines IL-1β, IL-6, and TNF-α, plus prostaglandins and the complement system. Several approved drugs block them.
• The CANTOS trial of 10,061 heart attack survivors showed that blocking IL-1β lowered cardiovascular events without lowering cholesterol, which supports a causal role for inflammation in atherosclerosis.
• Exercise, a Mediterranean-style diet, and adequate sleep are associated with lower inflammatory markers. The effects are real but modest.

A short history of the four signs

Celsus is credited with the four classic signs, written as rubor et tumor cum calore et dolore, meaning redness and swelling with heat and pain. The fifth sign is often attributed to Galen, but historians have found that attribution doubtful. A review in the medical literature notes that it "could have been proposed by either Thomas Sydenham or Rudolf Virchow," and Virchow is the name most often attached to it. Details like these matter less than the idea they record: a damaged or infected tissue changes in visible, predictable ways.

For centuries, doctors treated inflammation as something to suppress. The surgeon John Hunter argued in the 18th century that it "is not to be considered as a disease, but as a salutary operation." Élie Metchnikoff's work on phagocytes (cells that engulf microbes) in 1891 reinforced the point that the response is protective at its core. Both views survive in modern medicine: inflammation is a defense that sometimes needs to be switched off.

Acute inflammation: the emergency response

Think of acute inflammation as a city's emergency system. A fire alarm sounds, roads are cleared for fire trucks, crews arrive in order of speed, and afterward cleanup crews restore the street. The metaphor breaks down in one important way: in the body, the alarm, the traffic changes, and the cleanup are all run by the same set of cells and signals.

Detection

When tissue is damaged, local cells and sentinel immune cells sense trouble through pattern recognition receptors. The best known are the Toll-like receptors (TLRs), which recognize molecular signatures of bacteria and viruses as well as molecules released by injured cells. (The broader recognition system is covered in How the Immune System Recognizes Threats.)

Vessel changes: why you swell and turn red

Mast cells release histamine, which widens blood vessels and makes them more permeable. Prostaglandins and leukotrienes, lipid signals made from membrane fats via the arachidonic acid pathway, add to the effect. A review of the topic notes that prostaglandins contribute to redness, vascular permeability, pain, swelling, and fever. More blood flow produces redness and warmth, and leaky vessels let plasma and cells enter the tissue, producing swelling. Swelling presses on nerve endings, and the chemical signals sensitize them, which is the pain.

The cellular response

Neutrophils arrive first, within minutes to hours. They are the rapid-response force: they engulf bacteria (phagocytosis) and kill them with reactive oxygen compounds and enzymes. Monocytes arrive from the blood over hours to days and mature into macrophages, which clear debris and coordinate the next phase. A neutrophil can also die in a way that releases web-like structures called neutrophil extracellular traps, first described in 2004, that help trap and kill pathogens.

Cytokines: the messengers

Cytokines are small proteins that immune cells use to talk to each other and to the brain. Three matter most in this story.

Cytokine	Main actions	Related drug targets
IL-1β	Triggers fever, activates immune cells, prompts the liver to make acute-phase proteins	Canakinumab (anti-IL-1β antibody)
IL-6	Similar effects; stimulates liver production of C-reactive protein and antibody production	Tocilizumab (blocks the IL-6 receptor) used in rheumatoid arthritis and severe COVID-19
TNF-α	Drives local inflammation and fever; in excess contributes to septic shock	Adalimumab and other anti-TNF biologics

Fever is part of this system. IL-1β and TNF drive it, and it likely helps the body fight some infections (see Why Fevers Help Fight Infections).

Resolution is an active job

Inflammation does not simply run out of fuel. The body produces specialized pro-resolving mediators, including lipoxins, resolvins, protectins, and maresins, that stop neutrophil recruitment, encourage macrophages to switch to a repair mode, and prompt the clearance of dying cells (a process called efferocytosis). Resolvins, protectins, and maresins are made from omega-3 fatty acids. Lipoxin A4 is made from arachidonic acid, and the researcher Charles Serhan reported that aspirin favors its production.

Glucocorticoids, the family that includes the body's own cortisol, are potent shut-off signals as well. (See Cortisol and the Stress Response.) A growing view is that chronic inflammation can reflect failed resolution as much as excess activation, though the evidence is still developing.

A worked example: a splinter

Imagine a splinter in your finger. Within minutes, injured cells and resident macrophages release signals, and the area turns red and warm as vessels dilate. Within an hour or two, neutrophils crowd into the tissue. If bacteria came in with the splinter, the neutrophils engulf them, and pus, a mix of dead neutrophils and debris, may form. Over the next days, macrophages clear the wreckage, resolution signals rise, and fibroblasts and new vessels rebuild tissue. If this works, the finger is back to normal in about a week. (For the repair phase, see Why Wounds Scab Over.)

The complement system: inflammation's amplifier

Running in parallel to the cellular response is the complement system, a cascade of more than 30 plasma proteins. It does three jobs: it marks pathogens for destruction (opsonization), it punches holes in microbial membranes through the membrane attack complex, and it amplifies inflammation by recruiting more immune cells. Complement can be triggered by antibody-antigen complexes or by pathogen surfaces directly. The full cascade is described in Complement System: The Body's Cascade Defense.

Dysregulated complement contributes to autoimmune diseases such as systemic lupus erythematosus. Complement inhibitors are now approved for several rare conditions.

Acute versus chronic inflammation

The two types differ in more than duration.

Feature	Acute inflammation	Chronic low-grade inflammation
Trigger	Injury or infection	Persistent infection, obesity, autoimmunity, stress, aging, other
Onset	Minutes to hours	Gradual, often unnoticed
Main cells	Neutrophils, then macrophages	Macrophages, lymphocytes, other cells
Signal strength	High	Low but sustained
Symptoms	Redness, heat, swelling, pain, fever	Often none
Outcome	Resolution and repair	Tissue damage over years
How it is found	Obvious	Blood markers such as high-sensitivity CRP

Chronic inflammation: when the signal will not turn off

Chronic inflammation is typically marked by lower concentrations of inflammatory mediators than an acute response, but they persist for years. Major contributors include:

• Persistent low-grade infection. Chronic gum disease, for example, is associated with higher systemic inflammatory markers.
• Autoimmunity, where the immune system attacks self tissue.
• Obesity. Fat tissue, particularly the visceral fat around organs, is not metabolically inert. It secretes inflammatory cytokines including TNF-α and IL-6, and people with obesity tend to show raised CRP and IL-6 even when healthy otherwise. (See The Biology of Obesity and Metabolic Syndrome.)
• Chronic psychological stress, acting through the stress hormone system and the nervous system's effects on immune cell movement.
• Gut microbiome disruption. Altered microbial communities can weaken the gut barrier, letting bacterial lipopolysaccharide (LPS), a molecule from bacterial cell walls, reach the bloodstream and activate TLRs. (See The Gut Microbiome Explained.)
• Aging. Franceschi's 2000 paper in the Annals of the New York Academy of Sciences coined "inflammaging" for the low-grade inflammatory state that accumulates with age. Proposed drivers include senescent cells, mitochondrial dysfunction, and epigenetic change. (See The 12 Hallmarks of Aging Explained.)

Inflammation and disease: the connections

Cardiovascular disease

Atherosclerosis, the buildup of plaque in artery walls, is understood as an inflammatory process and not just a cholesterol deposit. Macrophages enter arterial walls, engulf oxidized LDL cholesterol, turn into foam cells, and secrete cytokines that feed plaque growth. (The cholesterol side is in Cholesterol Explained and Cardiovascular Disease and Atherosclerosis.)

The strongest evidence that inflammation is a cause, rather than a bystander, is the CANTOS trial, published in the New England Journal of Medicine in 2017. It enrolled 10,061 people who had previously had a heart attack and who had a high-sensitivity C-reactive protein level of at least 2 mg/L. They received placebo or canakinumab, an antibody that blocks IL-1β. Over a median follow-up of 3.7 years, the 150 mg dose cut the rate of the primary endpoint (nonfatal heart attack, nonfatal stroke, or cardiovascular death) with a hazard ratio of 0.85 compared with placebo, according to a Healio summary of the results. LDL and HDL cholesterol did not change. That is the point of the trial: the benefit came without a cholesterol effect.

CANTOS also showed the cost of blocking inflammation. Deaths from infection or sepsis were more common with canakinumab than with placebo (0.31 versus 0.18 per 100 person-years), and all-cause mortality did not differ between groups. The immune system's alarm is useful, and turning it down has a price.

A cheaper drug made the same point. In June 2023 the FDA approved Lodoco, a low-dose colchicine tablet, to reduce the risk of heart attack, stroke, coronary revascularization, and cardiovascular death in adults with established atherosclerotic disease or multiple risk factors. The approval rested on the LoDoCo2 trial of 5,522 patients, in which the manufacturer reported a 31% relative risk reduction, according to a HCPLive report. Colchicine has long been used at higher doses for gout.

Cancer

The link between chronic inflammation and cancer is long-standing. Rudolf Virchow is usually credited with first perceiving the connection, and a 2001 Lancet review by Balkwill and Mantovani revived it under the title "Inflammation and cancer: back to Virchow?" Their argument was that the inflammatory cells and cytokines found in tumors are more likely to promote growth and suppress effective immune attack than to defeat the tumor. They offered a metaphor: genetic damage is the match, and inflammation is the fuel.

Infections that cause chronic inflammation are a major real-world example. A study by IARC researchers in The Lancet Global Health estimated that 2.2 million new cancer cases in 2018, about 13% of the total, were attributable to infections. The leading causes were Helicobacter pylori (810,000 cases, mostly stomach cancer), human papillomavirus (690,000), hepatitis B virus (360,000), and hepatitis C virus (160,000), per the IARC summary. Not every one of those cancers works through inflammation, but chronic infection and inflammation are central to several. Inflammatory bowel disease also raises colorectal cancer risk. (See What Causes Cancer?.)

Neurodegeneration

The brain has its own resident immune cells, the microglia, which monitor for damage and infection. Chronically activated microglia release IL-1β, TNF-α, and complement proteins, and researchers link that activity to synapse loss and neuronal death in Alzheimer's disease, Parkinson's disease, and ALS. Amyloid plaques in Alzheimer's appear to do harm both physically and as inflammatory stimuli, though the relative weight of each effect is still being worked out. (See Alzheimer's Disease: The Biology Behind the Decline.)

Depression

The inflammatory theory of depression grew out of a clinical observation: interferon-alpha, a cytokine used to treat chronic hepatitis C, frequently caused depression as a side effect. Reviews describe depression as one of the most common side effects of the interferon-plus-ribavirin regimen. Researchers use it as a model for how inflammatory signals might alter mood. Some depressed patients have raised inflammatory markers. Whether inflammation causes depression, or shared factors such as stress and poor sleep cause both, remains partly open. (See Depression: The Neuroscience Behind It.)

When the response misfires: four patterns

Disease rarely looks like "too much inflammation" in a generic sense. It tends to fall into a few recognizable patterns, and the differences explain why treatments are not interchangeable.

A response aimed at the wrong target. In allergy, the immune system reacts strongly to a harmless substance such as pollen or a food protein. In autoimmune disease, the target is the body's own tissue. Rheumatoid arthritis, lupus, and multiple sclerosis all belong here, and the inflammation is real, only misdirected.

A response that is too large. In severe infection, the body can mount a systemic cytokine surge, sometimes called a cytokine storm. TNF-α in high concentrations contributes to the dangerous drop in blood pressure seen in septic shock. Here the problem is scale and spread: a local defense becomes a whole-body one. This pattern is also why drugs that block IL-6 were tried in severe COVID-19.

A response that does not end. Persistent infection, a continuing irritant such as smoke, or a failure of the resolution program can keep the process running at low intensity. This pattern fits atherosclerosis best, where plaque keeps drawing in macrophages for decades.

A response driven by metabolic or age-related signals. In obesity and aging, there may be no single trigger. Instead, stressed fat cells, senescent cells, and a leakier gut barrier feed a hum of inflammatory signaling. This is the least well understood of the four, and the one where public claims most often outrun the data.

The patterns overlap. A person with obesity can also have an autoimmune condition, and a chronic infection can set the stage for a cancer. Still, the framework helps explain why a drug that works in rheumatoid arthritis may do nothing for heart disease, and why the colchicine and canakinumab results were treated as news. They suggested that, for atherosclerosis, a drug aimed at the inflammatory process could add to the benefit of cholesterol control.

How inflammation is measured

Acute inflammation announces itself. Chronic low-grade inflammation usually does not, and it is detected through blood tests. The most common is high-sensitivity C-reactive protein (hsCRP). CRP is made by the liver in response to IL-6.

A 2003 statement from the American Heart Association and the Centers for Disease Control and Prevention set the commonly used cardiovascular risk bands.

hsCRP level	Interpretation
Below 1.0 mg/L	Lower cardiovascular risk
1.0 to 3.0 mg/L	Average risk
Above 3.0 mg/L	Higher risk
Above 10 mg/L	Suggests acute inflammation or infection; retest after recovery

Lab notes citing that statement also advise averaging two measurements taken at least two weeks apart. A single high result after a cold or a hard workout tells you little. Other markers, including IL-6, fibrinogen, ferritin, and white blood cell count, appear in research and in clinical workups, but hsCRP is the standard one for cardiovascular risk. A raised hsCRP is a risk marker, not a diagnosis.

What measurably changes inflammatory markers

The word "measurably" matters. Most of these findings are about markers such as CRP, and effect sizes are modest.

Exercise. A 2017 meta-analysis in the British Journal of Sports Medicine by Fedewa and colleagues pooled 83 trials and found that exercise training lowered CRP, with a small average effect (an effect size of 0.26). Reductions were larger when body weight or body fat also dropped. Muscles release IL-6 during exercise, but in that setting it works as a signal that promotes anti-inflammatory pathways, which is one reason the same molecule can look like a villain in disease and a messenger in exercise. Sedentary behavior is associated with higher CRP. (See The Science of Exercise.)

Diet. A 2014 meta-analysis of 17 randomized trials, covering about 2,300 participants, found that people on Mediterranean-style diets had a significantly greater fall in hsCRP than people on comparison diets, an average difference of about 1 mg/L (PubMed). The trials differed widely, so the pooled number is better read as a direction than a prediction. Diets of this type are rich in vegetables, fruit, legumes, fish, and olive oil, and low in ultra-processed food.

Sleep. A meta-analysis by Irwin and colleagues in Biological Psychiatry reviewed 72 studies with more than 50,000 participants. It found that poor sleep quality was associated with higher CRP and IL-6, and short sleep with higher CRP, while the study found no link with TNF-α. Very long sleep was also associated with higher markers, which probably reflects underlying illness in some people. These are associations. (PMC) (See The Science of Sleep.)

Smoking. Smoking is a pro-inflammatory exposure, and quitting removes it.

None of these replaces treatment for an established inflammatory disease. They shift risk at the margin.

How anti-inflammatory drugs differ

"Anti-inflammatory" covers several unrelated mechanisms.

Drug class	Examples	How it works	Typical use
NSAIDs	Ibuprofen, naproxen	Inhibit COX enzymes, reducing prostaglandins	Pain, fever, short-term inflammation
Corticosteroids	Prednisone	Broadly suppress immune signaling, including through NF-κB	Flares of autoimmune and allergic disease
Biologics	Adalimumab, tocilizumab, canakinumab	Antibodies against specific cytokines or receptors	Rheumatoid arthritis, other autoimmune disease
JAK inhibitors	Tofacitinib and others	Block signaling inside immune cells	Selected autoimmune diseases
Colchicine	Gout formulations; low-dose Lodoco	Dampens inflammatory activity of neutrophils and related pathways	Gout; cardiovascular risk reduction (low dose)

Each class carries different risks, and the immune suppression that helps in one disease can raise infection risk, as CANTOS showed. These choices belong to a clinician.

Common misunderstandings

"All inflammation is bad." Without it, wounds and infections would be fatal. The problem is inflammation that is excessive, mistimed, or never resolved.

"Inflammation causes every disease." Headlines overstate it. Inflammation is a contributor in many conditions, but it is one factor among genetics, metabolism, infection, and environment, and in some conditions its role is still debated.

"A high CRP means you have a particular disease." CRP rises with infections, injuries, and many other causes. It is a nonspecific signal.

"There is an anti-inflammatory superfood." The research supports dietary patterns more than single ingredients. Wellness marketing often outruns the evidence.

"Lowering inflammation is risk-free." CANTOS found more fatal infections with canakinumab. Inflammation protects you.

FAQ

Q: Are all anti-inflammatory drugs doing the same thing? A: No. NSAIDs inhibit cyclooxygenase (COX) enzymes, which reduces prostaglandin production. Corticosteroids broadly suppress immune signaling, including through the transcription factor NF-κB. Biologics such as adalimumab and tocilizumab use antibodies to block specific cytokines or their receptors, and JAK inhibitors block signaling inside cells. Each has different benefits, side effects, and appropriate uses.

Q: Is "anti-inflammatory eating" a real thing? A: In a limited sense, yes. Dietary patterns such as the Mediterranean diet are associated with lower inflammatory markers, and a pooled analysis of randomized trials found an average hsCRP reduction of about 1 mg/L. The effect is modest, and diet is not a substitute for treating established inflammatory disease. The pattern matters more than any single food.

Q: Can inflammation be detected through symptoms? A: Acute inflammation is obvious: redness, heat, swelling, and pain. Chronic low-grade inflammation is usually silent and shows up only in blood tests such as hsCRP. An unexplained, persistently raised hsCRP is a cardiovascular risk marker worth discussing with a clinician, who will also check for infection and other causes.

Q: Is "leaky gut" real? A: Intestinal permeability is real. The tight junctions between gut lining cells can be loosened by certain infections, inflammatory disease, alcohol, and possibly some dietary components. Whether mild increased permeability drives systemic inflammation in otherwise healthy people is more contested, and evidence for supplements that claim to fix it is weak.

Q: Why do some immune systems attack the body's own tissues? A: Autoimmunity reflects a failure of immune tolerance. Developing T and B cells that react strongly to self are normally eliminated, and peripheral mechanisms restrain those that escape. Genetic variants, especially in HLA genes, combined with environmental triggers such as infections or microbiome changes, can break that tolerance. Targets include joints in rheumatoid arthritis, the thyroid in Hashimoto's disease, and the nervous system in multiple sclerosis.

Q: Does fever count as inflammation? A: Yes, it is the whole-body version. Cytokines such as IL-1β and TNF-α signal the brain to raise the temperature set point, and the higher temperature appears to help immune defenses against some pathogens.

The takeaway

Inflammation is best understood as a system with an on switch and an off switch, and disease tends to arise when either one fails. A missing response lets infection spread. A response that never resolves can damage arteries, brain tissue, and DNA over years, which is why a trial that quieted one cytokine could reduce heart attacks without touching cholesterol. The same trial found more fatal infections, a reminder that the goal is a better-regulated response, not a silent one. The most practical lesson from current evidence is modest: movement, sleep, and a varied diet nudge the markers in the right direction, and medicines that target inflammation belong in a doctor's hands.

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Read next:

• How Vaccines Work: The Immune Response Behind Immunization
• Epigenetics: How Your Environment Changes the Way Your DNA Is Read
• The Gut Microbiome Explained: How Bacteria Shape Your Health
• The Immune System: How It Works Explained