What Is Inflammation? Causes, Types, and Treatment

Inflammation is the body’s built-in alarm and repair system, a cascade of immune responses triggered whenever tissues are damaged or threatened by infection, injury, or irritation. In its acute form it is fast, targeted, and self-limiting: blood vessels widen, immune cells rush in, and the area swells, reddens, heats up, and hurts until the threat is neutralized and healing begins. When this process fails to shut off, or fires without a clear external threat, it becomes chronic inflammation, a quieter, body-wide smolder linked to heart disease, neurodegeneration, diabetes, and cancer. Understanding the difference between these two faces of inflammation, what sets them off, and what can be done about each is central to making sense of modern medicine.

What Happens During Acute Inflammation

When you cut your finger, sprain an ankle, or catch a cold, your immune system launches a rapid response. Within minutes, blood vessels near the site of injury become more permeable, allowing fluid, proteins, and immune cells to flood into the surrounding tissue.¹ That extra fluid is what causes the swelling and warmth you feel. The redness comes from increased blood flow, and the pain comes from chemical signals that sensitize local nerve endings, a useful reminder not to keep using the injured area.

The first immune cells on the scene are neutrophils, fast-moving white blood cells that engulf bacteria and debris. They dominate the early stages and essentially prepare the ground for the next wave: macrophages, which arrive to clean up dead cells and begin coordinating tissue repair.² This handoff from neutrophils to macrophages is one of the most critical transitions in the whole process. If it goes smoothly, healing follows. If it stalls, the inflammation can drag on and start damaging the very tissues it was supposed to protect.

How Inflammation Is Supposed to End

One of the least appreciated aspects of the inflammatory response is that your body actively shuts it down. Resolution is not passive; it does not happen simply because the threat is gone. The body produces a class of signaling molecules derived from omega-3 and omega-6 fatty acids, collectively called specialized pro-resolving mediators. These include lipoxins, resolvins, protectins, and maresins, each with a distinct chemical structure and role in telling immune cells to stand down, stop recruiting reinforcements, and switch into cleanup mode.³

Research in animal models has shown that synthetic versions of these molecules can reduce inflammation and even help the body clear infections more effectively, because resolution and antimicrobial defense are not opposed processes. They work together.⁴ When this resolution machinery is impaired, whether because of nutrient deficiencies, genetic factors, or ongoing stimuli that keep the alarm blaring, acute inflammation can slide into a chronic state. That transition from helpful to harmful is where most of the trouble begins.

Common Causes and Triggers

Acute inflammation has straightforward triggers: a bacterial or viral infection, a burn, a broken bone, a splinter. The cause is obvious, the response is proportional, and the whole cycle wraps up in days to weeks. Chronic inflammation is different. Its triggers are subtler, often multiple, and frequently tied to modern lifestyle.

Excess Body Fat

Adipose tissue is not just a passive energy reservoir. In obesity, fat cells become dysfunctional, secreting inflammatory signaling molecules and attracting immune cells that take up residence in fat deposits and keep pumping out inflammatory signals.⁵ This immune infiltration occurs not just in fat tissue but across multiple organs, including skeletal muscle, liver, and the brain, and contributes to insulin resistance and the development of type 2 diabetes.⁶

Gut Barrier Disruption

The lining of your intestine is a single-cell-thick barrier that decides what gets into the bloodstream and what stays out. When that barrier is compromised, sometimes called “leaky gut,” bacterial products like lipopolysaccharide can escape into circulation and provoke a systemic inflammatory response. Factors that damage this barrier include high-fat diets, chronic alcohol use, oxidative stress, and imbalances in gut bacteria.⁷ This pathway appears to be connected to the progression of several metabolic and autoimmune conditions.

Chronic Psychological Stress

Cortisol, the body’s primary stress hormone, normally acts as an anti-inflammatory brake. But under prolonged stress, the system can malfunction. The immune system’s receptors for cortisol become less responsive, a phenomenon that leaves the body unable to properly dial down inflammation even when cortisol levels are elevated.⁸ This dysregulation creates a paradox where the body simultaneously produces stress hormones and inflammatory signals, promoting a state that favors autoimmune activity and chronic disease.⁹ The same process has been implicated in the progression of neurodegenerative conditions like Alzheimer’s and Parkinson’s disease, as well as depression.¹⁰

Air Pollution and Environmental Toxins

Even relatively low levels of ambient air pollution can move the needle on inflammation. Data from the Framingham Heart Study found that short-term increases in air pollution exposure were associated with higher blood levels of C-reactive protein and interleukin-6, two well-known inflammatory markers, in people living in the greater Boston area.¹¹ This finding is concerning because it means that inflammation from environmental sources does not require dramatic exposure events; everyday air quality plays a role.

When Inflammation Drives Disease

Chronic low-grade inflammation is not a disease itself but a condition that accelerates many diseases. The connections have become clearer over the past two decades, and a few of the most important ones are worth understanding in detail.

Heart Disease and Atherosclerosis

Atherosclerosis, the buildup of fatty plaques in arteries, is now understood to be fundamentally an inflammatory disease.¹² Macrophages that enter the artery wall to deal with deposited cholesterol end up gorging on fats and transforming into foam cells, which form the unstable core of arterial plaques.¹³ When a plaque ruptures, the resulting clot can block blood flow and cause a heart attack or stroke. In this sense, a heart attack is not just a plumbing problem; it is an inflammatory event years in the making.

Neurodegeneration

The brain has its own resident immune cells called microglia. Under healthy conditions, microglia clear waste products and protein aggregates. But when chronically activated, they can shift into a state that damages neurons instead of protecting them. Excessive uptake of abnormal protein clumps can impair microglial function, leading to sustained neuroinflammation that contributes to conditions like Alzheimer’s and Parkinson’s disease.¹⁴ Research suggests that modulating microglial behavior could eventually become a treatment strategy for multiple neurodegenerative diseases.¹⁵

Cancer

The link between inflammation and cancer has been recognized for over a century, but the molecular details have only become clear recently. A key player is a signaling pathway called NF-κB, which is frequently activated in tumors. In cancer cells, it switches on survival genes that help the tumor resist destruction. In the surrounding tissue, it promotes the production of inflammatory molecules that create a tumor-friendly environment.¹⁶ This is why chronic inflammatory conditions like ulcerative colitis and hepatitis carry elevated cancer risks in the affected tissues.

Autoimmune Versus Autoinflammatory Conditions

Not all inflammation gone wrong is the same. Autoimmune diseases like rheumatoid arthritis and lupus involve the adaptive immune system, meaning the body’s B cells and T cells mistakenly target its own tissues, producing autoantibodies and mounting attacks as if healthy cells were foreign invaders. Autoinflammatory diseases, by contrast, are driven by the innate immune system, the more ancient, blunt-instrument branch. Conditions like familial Mediterranean fever and gout involve excessive activation of innate pathways without the specific antibody targeting seen in autoimmunity.¹⁷

The distinction matters for treatment. Autoimmune diseases often respond to drugs that suppress the adaptive immune system, while autoinflammatory conditions may need therapies targeting innate immune pathways like interleukin-1. In practice, many diseases sit on a spectrum between the two, with elements of both innate and adaptive immune dysfunction. Researchers increasingly recognize that even conditions historically classified as autoimmune, like multiple sclerosis, have substantial autoinflammatory components.

Measuring Inflammation

If chronic inflammation is so central to disease, can you just get a blood test for it? You can, but interpreting the results is trickier than it seems. The two most commonly ordered markers are C-reactive protein (CRP) and interleukin-6 (IL-6), and both have quirks that make them less straightforward than they are often treated.

IL-6 operates through two distinct signaling pathways. Only one of those pathways is specifically activated during inflammation; the other influences a range of normal body processes even in healthy people. Similarly, CRP comes in two forms. One is produced locally in damaged tissues and reflects genuine inflammation. The other is routinely made by the body without any inflammation and may even have anti-inflammatory effects.¹⁸ So an elevated CRP on a blood test does not always mean what you think it means.

Head-to-head comparisons of the two markers suggest that IL-6 has better diagnostic accuracy for detecting systemic inflammation than CRP does, and that tracking IL-6 over time can help predict complications in both acute and chronic conditions.¹⁹ Still, neither marker alone gives a complete picture. Clinical decisions about inflammation usually rely on patterns across multiple markers, imaging, and symptoms rather than a single number.

Pharmacological Treatments

Anti-inflammatory drugs span a wide range, from cheap over-the-counter pills to sophisticated injectable biologics. Which one makes sense depends entirely on what kind of inflammation you are dealing with.

NSAIDs and COX Inhibitors

Nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen work by blocking an enzyme involved in producing prostaglandins, the chemical signals that promote swelling and pain. This enzyme exists in two forms: one involved in normal housekeeping functions like protecting the stomach lining, and another that ramps up during inflammation. Standard NSAIDs block both forms, which explains why they can cause stomach irritation and other side effects. Selective COX-2 inhibitors were designed to target only the inflammation-related form, preserving more of the protective function.²⁰ NSAIDs are workhorses for acute pain and conditions like osteoarthritis, where short-term symptom control is the goal.²¹

Corticosteroids

When inflammation is severe or widespread, corticosteroids like prednisone and dexamethasone offer much more potent suppression than NSAIDs. They work broadly across the immune system, dialing down many inflammatory pathways simultaneously. The trade-off is that long-term use comes with substantial side effects: bone thinning, weight gain, elevated blood sugar, increased infection risk. For this reason, doctors typically use corticosteroids at the lowest effective dose for the shortest possible time, or inject them directly into a joint or tissue to minimize whole-body exposure.

Biologic Therapies

The biggest advance in treating chronic inflammatory diseases over the past two decades has been the development of biologic drugs, engineered proteins that target specific molecules in the inflammatory cascade. The earliest and most widely used are TNF-alpha blockers, which have proven effective across a growing list of conditions including rheumatoid arthritis, Crohn’s disease, psoriasis, and ankylosing spondylitis.²² More recent additions include drugs targeting IL-1, IL-6 receptors, and IL-17, each aimed at different nodes in the inflammatory network.²³ Biologics have been transformative for conditions like inflammatory bowel disease, allowing directed treatment at the molecules driving disease rather than blanketing the entire immune system.²⁴

Their main limitations are cost, the need for injection or infusion, and the fact that suppressing specific immune pathways can increase susceptibility to certain infections. Not everyone responds to the same biologic, either, so treatment often involves trial and error before finding the right fit.

Lifestyle and Dietary Approaches

Medication is not the only lever. Several lifestyle factors have measurable effects on inflammatory markers, and for people with low-grade chronic inflammation rather than a diagnosed inflammatory disease, they are often the most practical starting point.

Exercise

Contracting muscles release signaling molecules called myokines into the bloodstream. One of the most studied is IL-6, which in this context plays a different role than it does during infection. Muscle-derived IL-6 triggers anti-inflammatory cascades, promotes fat burning, improves glucose uptake, and can counteract the inflammatory signaling that comes from excess body fat.²⁵ This is one reason why regular physical activity reduces inflammation even when it does not result in weight loss, and why skeletal muscle is increasingly recognized as an immune-regulating organ in its own right.²⁶

Omega-3 Fatty Acids

Omega-3 fats, found in fatty fish, flaxseed, and walnuts, serve as the raw material for those pro-resolving mediators discussed earlier. Supplementation has been shown to lower CRP and prostaglandin E2, with meaningful effect sizes in populations with elevated baseline inflammation like heavy smokers.²⁷ What may matter as much as total omega-3 intake is the ratio of omega-6 to omega-3 fats in the diet. Higher ratios have been associated with higher CRP levels, suggesting that rebalancing the two, rather than simply adding omega-3, is important.²⁸

Sleep and Circadian Rhythms

The immune system runs on a clock. Inflammatory molecules, immune cell counts, and cytokine production all follow circadian rhythms, peaking and ebbing at predictable times of day. When that clock is disrupted by shift work, chronic sleep deprivation, or irregular schedules, immune regulation suffers.²⁹ This is one mechanism behind the association between poor sleep and higher rates of cardiovascular disease, metabolic problems, and even neurodegeneration.

Aging and Inflammaging

As people age, the body develops a baseline level of chronic, low-grade inflammation even in the absence of infection or obvious disease. Researchers have given this phenomenon its own name: inflammaging. It is driven in part by the accumulation of senescent cells, older cells that stop dividing but refuse to die. Instead of quietly retiring, these cells adopt what is called a senescence-associated secretory phenotype, continuously releasing inflammatory cytokines, growth factors, and enzymes that remodel surrounding tissue.³⁰

This slow-burn inflammation contributes to the development of age-related cardiovascular diseases, including atherosclerosis and coronary heart disease.³¹ It also compounds the effects of other inflammatory drivers. A 70-year-old with obesity, chronic stress, and poor sleep is not simply adding those risk factors together; the inflammatory load from each source feeds into and amplifies the others. This is part of why interventions that seem modest in isolation, like regular walking or improved sleep hygiene, can have outsized effects in older adults: they are chipping away at a compounding problem.

Why Modern Life Fans the Flames

Viewed through an evolutionary lens, the mismatch between our inflammatory machinery and our environment is striking. Our immune systems evolved in a world of frequent infections, parasites, physical trauma, and caloric scarcity. An aggressive inflammatory response was a survival advantage in that context. But in modern environments characterized by abundant food, sedentary routines, chronic psychological stress, and low pathogen exposure, many of those same inflammatory pathways fire without a meaningful threat to fight.³²

This mismatch theory helps explain why diseases driven by chronic inflammation, including type 2 diabetes, cardiovascular disease, and autoimmune conditions, have become so common in industrialized populations while remaining rarer in societies with more traditional lifestyles.³³ The problem is not that our immune systems are broken. They are doing exactly what they were designed to do, just in a world that no longer matches the design.