Inflammation is your body’s built-in alarm and repair system. When tissue is damaged or a pathogen gets in, your immune system launches an inflammatory response to contain the threat, clear out debris, and begin healing. It’s not a malfunction. It’s one of the most fundamental survival mechanisms you have. The trouble starts only when this process doesn’t shut off properly.
What Triggers the Response
Your body is constantly monitoring for two categories of danger. The first is foreign invaders: bacteria, viruses, fungi, and other pathogens that carry molecular signatures your immune cells recognize as threats. The second is tissue damage itself. When your own cells are injured or stressed, whether from a cut, a burn, a torn muscle, or even intense physical pressure, they release molecules that act as distress signals. These molecules don’t trigger any immune response under normal conditions, but when cells are damaged, their concentration spikes or their chemical properties change, and your immune system reads that as a call to action.
Both categories of trigger activate the same basic machinery. Immune cells stationed throughout your tissues detect the danger signals, and within seconds they start releasing chemical messengers that kick off a cascade of changes in the surrounding blood vessels and tissue.
What Happens Inside Your Body
The inflammatory response unfolds in a rapid, coordinated sequence. First, blood vessels near the injury dilate, increasing blood flow to the area. That’s what causes the redness and warmth you feel around a wound or infection. Then the walls of those blood vessels become more permeable. The cells lining the vessels essentially become “leaky,” allowing fluid, proteins, and immune cells to escape from the bloodstream into the surrounding tissue. This flood of fluid is what produces swelling.
Several chemical mediators orchestrate these changes. Prostaglandins are potent vasodilators, meaning they widen blood vessels and are produced in large enough quantities at inflammatory sites to account for the characteristic redness. They also play a key role in inflammatory pain by sensitizing nerve endings. Histamine and bradykinin increase vascular permeability on their own, but prostaglandins dramatically amplify their effects on swelling. These chemicals work together, each one boosting the impact of the others.
The result is the five classic signs of inflammation, described in medical texts for centuries by their Latin names: pain (dolor), heat (calor), redness (rubor), swelling (tumor), and loss of function (functio laesa). Each one traces back to these vascular and chemical changes. Pain comes from both the pressure of swelling on nerve endings and the direct sensitization of those nerves by prostaglandins. Loss of function is partly mechanical (a swollen joint simply can’t move as well) and partly protective, discouraging you from using the injured area.
How Immune Cells Reach the Scene
Once blood vessels have opened up, the next step is getting the right immune cells to the exact spot they’re needed. White blood cells circulating in the bloodstream go through a multi-step process to leave the vessel and enter the tissue. They first slow down by rolling along the inner wall of the blood vessel, loosely sticking and unsticking to its surface. Then they firmly attach. After locking on, they reduce their grip just enough to crawl along the vessel wall, searching for a suitable exit point. Finally, they squeeze between or through the endothelial cells lining the vessel and push into the tissue beyond.
In the early hours of acute inflammation, the first responders are neutrophils, fast-acting immune cells that engulf and destroy bacteria and damaged cells. Macrophages follow, serving as both destroyers and cleanup crews. They consume pathogens, dead cells, and debris, and they also release signaling molecules that coordinate the next phases of the response.
When Inflammation Goes Systemic
Most inflammation stays local. A splinter in your finger produces redness and swelling at the site, and the response stays contained. But when the threat is large enough, or widespread enough, inflammation can produce effects throughout the entire body.
Activated macrophages and other immune cells release signaling molecules called cytokines, including TNF-alpha, IL-1, and IL-6. These travel through the bloodstream and trigger what’s known as the acute phase response. They stimulate the liver to produce special proteins that help fight infection. They also act on the brain’s temperature-regulating center, which is why infections often cause fever. Fever isn’t a symptom of the pathogen itself. It’s your immune system deliberately raising your body temperature to create a less hospitable environment for invaders and to speed up immune cell activity.
How the Body Turns Inflammation Off
A healthy inflammatory response is self-limiting. Your body doesn’t just wait for inflammation to burn out on its own. It actively produces specialized molecules that signal the process to wind down. This resolution phase involves a complex mix of anti-inflammatory cytokines, metabolic signals, and a class of compounds called specialized pro-resolving mediators, or SPMs.
SPMs are produced on demand from omega-3 fatty acids like EPA and DHA (the same fats found in fish oil and fatty fish). They work at extremely low concentrations, binding to receptors on immune cells to shift their behavior from attack mode to cleanup mode. Specifically, they enhance the ability of macrophages to clear away dead cells and debris, a process essential for tissue repair. They also fine-tune the activity of prostaglandins, the same molecules that drove the early inflammatory response, redirecting their signaling toward healing rather than alarm.
Resolution isn’t just the tail end of inflammation. It’s better understood as a distinct, active mid-phase in a highly coordinated sequence that also involves nerve signals, gases like nitric oxide, and various proteins. When this resolution machinery works correctly, you go from injury to healing to restored tissue in a matter of days.
Why Inflammation Sometimes Won’t Stop
Chronic inflammation occurs when the resolution phase fails. According to the National Cancer Institute, the three most common reasons are infections that don’t fully clear, abnormal immune reactions to normal tissues (as in autoimmune diseases), and conditions like obesity.
Each of these creates a situation where the immune system never gets the “all clear” signal. A lingering low-grade infection keeps triggering new waves of immune activation. In autoimmune conditions like rheumatoid arthritis or lupus, the immune system mistakenly identifies the body’s own tissues as threats, so the trigger never goes away. Obesity contributes because fat tissue, particularly visceral fat around the organs, is metabolically active and continuously releases inflammatory signaling molecules. The immune system responds as though there’s an ongoing injury, even when there isn’t one.
The cell types involved also shift. Acute inflammation is dominated by neutrophils, which arrive fast, do their job, and die off quickly. Chronic inflammation involves longer-lived cells like macrophages and lymphocytes that sustain the response over weeks, months, or years. Over time, this persistent low-level inflammation damages healthy tissue and is now recognized as a contributing factor in heart disease, type 2 diabetes, certain cancers, and neurodegenerative conditions.
The difference between acute and chronic inflammation is essentially the difference between a fire that’s lit to clear brush and one that escapes the firebreak. The biology is the same. The outcome depends entirely on whether the body can shut it down when the job is done.