Inflammation is caused by your immune system responding to anything it perceives as a threat. That threat can be a bacterial infection, a sprained ankle, excess body fat, psychological stress, or even air pollution. The immune system uses the same basic playbook each time: it detects something wrong, sends immune cells to the site, and releases chemical signals that produce swelling, heat, redness, or pain. What varies is whether the response shuts off after a few days or lingers for months or years.
How the Immune System Detects Threats
Your body’s first line of defense relies on pattern recognition receptors, sensors embedded in immune cells that scan for two categories of danger signals. The first category comes from invaders like bacteria, viruses, and fungi. These organisms carry molecular signatures that human cells don’t have, and your immune sensors are built to spot them. A receptor called TLR4, for example, detects a molecule found on the outer membrane of certain bacteria. TLR3 recognizes genetic material from RNA viruses. TLR2 picks up components from a wide range of microbes, including yeast and gram-positive bacteria.
The second category of danger signals comes from your own damaged cells. When tissue is injured by a burn, a cut, blunt force, or a toxin, dying cells spill their internal contents into the surrounding area. These molecules, called damage-associated molecular patterns, trigger the same immune receptors that infections do. This is why a twisted knee swells up even without any germs involved. Your immune system treats the tissue damage itself as a reason to mount a response.
Infections: The Classic Trigger
Bacterial, viral, and fungal infections are the most straightforward cause of inflammation. When bacteria enter a wound, immune cells recognize structural components like the whip-like flagella bacteria use to move, or the unique sugar-fat molecules that make up their outer walls. Viruses trigger a different set of sensors tuned to detect foreign genetic material, whether single-stranded or double-stranded RNA. Fungal infections activate yet another set of receptors.
Once detected, the immune system floods the area with white blood cells and releases signaling molecules that widen blood vessels, increasing blood flow to the site. This is what produces the familiar redness, warmth, swelling, and tenderness of an infected cut or a sore throat. In a healthy response, the infection is cleared and the inflammation resolves within days to a couple of weeks.
Physical Injury and Burns
Trauma, surgery, burns, and frostbite all cause what researchers call sterile inflammation, meaning there’s no pathogen involved. The trigger is purely mechanical or thermal damage to cells. As cells rupture, they release internal molecules that neighboring immune cells interpret as alarm signals. Macrophages, a type of immune cell that acts as both sensor and cleanup crew, arrive quickly and shift into an inflammatory mode.
Interestingly, dying cells don’t just release pro-inflammatory signals. They simultaneously produce resolution-associated molecules that help dial down the response once the damage is contained. The balance between these two sets of signals determines whether healing proceeds smoothly or whether inflammation becomes prolonged. Burns and major trauma tend to produce intense, widespread versions of this response, sometimes affecting organs far from the original injury site.
Excess Body Fat
One of the most significant sources of chronic, low-grade inflammation in modern life is excess adipose tissue. Fat cells are not passive storage units. They actively secrete signaling molecules, and as fat mass increases, the chemical profile of those signals shifts toward inflammation. Expanding fat tissue attracts immune cells, particularly macrophages that produce a powerful inflammatory molecule called TNF-alpha. These macrophages become a major, ongoing source of inflammatory signaling throughout the body.
As obesity progresses, immune cells are further recruited through chemical messaging pathways, turning the inflammation into a self-sustaining chronic process. This persistent, body-wide inflammation is a key link between obesity and conditions like type 2 diabetes, heart disease, and certain cancers. The inflammatory molecules released by fat tissue directly interfere with insulin signaling, which is one reason excess weight makes blood sugar harder to regulate.
Diet and Inflammatory Foods
What you eat can either fuel or dampen inflammation. Research using dietary inflammation scoring systems has consistently identified several food categories that raise inflammatory markers in the blood. The main culprits include refined grains and starches (white bread, white rice, pastries), processed meats (bacon, sausage, deli meats), red meat in large quantities, foods with added sugars, and certain added fats, particularly trans fats found in some fried and packaged foods. High-energy sweetened beverages also rank as pro-inflammatory.
These foods don’t cause the dramatic, immediate inflammation you’d see with an infection. Instead, they contribute to the kind of subtle, persistent elevation in inflammatory markers that accumulates over years and raises the risk of chronic disease.
Stress, Sleep, and Sedentary Habits
Chronic psychological stress drives inflammation through hormonal pathways. Stress hormones, when elevated over long periods, alter immune cell behavior in ways that promote inflammatory signaling rather than suppress it. This is one reason prolonged stress is linked to conditions ranging from heart disease to depression.
Sleep deprivation compounds the problem. Adults who consistently get less than seven hours of sleep per night show elevated inflammatory markers compared to those sleeping seven to nine hours. Poor sleep weakens the immune system’s normal regulatory functions, making it more likely to produce inappropriate inflammatory responses. A sedentary lifestyle adds a third layer. Regular physical activity, roughly 150 minutes of moderate exercise per week, helps regulate hormones, maintain healthy body weight, and directly reduces circulating inflammatory molecules. Sitting all day removes that protective effect.
Autoimmune Diseases
In autoimmune conditions like rheumatoid arthritis, lupus, type 1 diabetes, and multiple sclerosis, the immune system attacks the body’s own tissues. This happens because of a breakdown in the quality-control process that normally weeds out dangerous immune cells. During immune cell development, the thymus gland tests immature immune cells against proteins found throughout the body. Any cell that reacts strongly to the body’s own proteins is destroyed before it can enter the bloodstream. When this screening process fails, self-reactive immune cells escape into circulation and can target joints, nerves, the pancreas, or other organs.
Several defects can cause this failure. The screening protein that orchestrates the process can carry mutations, allowing dangerous immune cells to slip through. Regulatory immune cells that normally keep the rest of the immune system in check can be reduced in number or function. The result is ongoing, tissue-destructive inflammation that persists because the “threat” the immune system is reacting to, your own body, never goes away.
Environmental Pollutants
Air pollution, heavy metals, and industrial chemicals are increasingly recognized as inflammatory triggers. Fine particulate matter (PM2.5) from vehicle exhaust and industrial emissions, formaldehyde in building materials, cadmium and mercury from contaminated water and soil, and polychlorinated biphenyls from industrial waste all activate inflammatory pathways in the body.
These pollutants work largely through oxidative stress, a state where harmful reactive molecules overwhelm the body’s ability to neutralize them. This imbalance activates a specific inflammatory complex inside cells called the NLRP3 inflammasome, which triggers a cascade of inflammatory signaling. Heavy metal accumulation has been specifically linked to neurological damage through this pathway. The inflammation caused by environmental exposures is typically chronic and low-level, building over years of exposure rather than producing obvious acute symptoms.
Aging and Cellular Wear
Aging itself is an independent driver of inflammation, a phenomenon researchers have termed “inflammaging.” As you age, cells throughout your body accumulate damage and eventually enter a state called senescence, where they stop dividing but don’t die. These senescent cells become factories for inflammatory molecules, steadily releasing cytokines, chemokines, and tissue-degrading enzymes into their surroundings. This cocktail of secreted molecules promotes local and systemic inflammation while also pushing neighboring healthy cells toward senescence, creating a self-amplifying cycle.
The consequences ripple through the immune system itself. Inflammatory signals from senescent cells impair the production of new immune cells, reducing the body’s ability to mount fresh immune responses while simultaneously driving chronic, unfocused inflammation. This is why older adults often experience both increased susceptibility to infections and higher rates of inflammatory diseases like atherosclerosis, arthritis, and neurodegeneration.
How Inflammation Is Measured
The most common blood test for inflammation is C-reactive protein (CRP), a protein the liver produces in response to inflammatory signals. For detecting acute inflammation from infections or major tissue injury, a standard CRP test is used. Results of 10 mg/L or higher are considered high and suggest significant active inflammation.
For detecting the subtler, chronic inflammation linked to heart disease and metabolic conditions, a high-sensitivity version of the test (hs-CRP) measures much smaller variations. An hs-CRP below 2.0 mg/L is associated with lower cardiovascular risk, while 2.0 mg/L or above signals higher risk. Newer markers like soluble urokinase plasminogen activator receptor (suPAR) are proving more reliable than CRP for tracking systemic chronic inflammation, though they aren’t yet as widely available in routine testing.