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In my last blog post, I described the major players in the immune system. The key point was that billions of immune cells and trillions of proteins carry out most of the work of the immune system, while organs and bone marrow play more of a supporting role (birthing, development, storing, and providing an expressway throughout the body).

In this post I’ll describe the “immune response”—i.e., what happens when the immune cells and proteins perceive a danger to the body and go to work to eliminate the danger—and how this translates into inflammation.

What happens when the immune response is triggered

The immune response starts when resident immune cells react to an enemy or irregularity in their proximity. The immune cells immediately send out protein signals, and a number of processes begin. Blood flow to the area increases. Blood vessels become more permeable, allowing immune and other cells, proteins, and fluids to cross more easily from the bloodstream into neighboring tissues. The immune cells begin to attack suspicious bodies, release chemicals to call more immune cells to the scene, and clean up damaged tissue and waste. Many different types of immune cells and proteins work together in complex ways to carry out these tasks.

This immune response is much the same regardless of the trigger, the type of perceived threat, the location in the body, and the scale of the threat. In all these cases, blood flow and fluids rise, vessels become more permeable, immune cells attack and release chemicals, proteins carry messages, other immune cells dispose of dead tissues and debris, and the immune system generates additional immune cells if needed. Whether the immune response lasts only a few days (called “acute inflammation”) or for weeks, months, or years (called “chronic inflammation”), the immune response takes place with virtually the same mechanisms.

Consequence of the immune response: Inflammation

The immune response has consequences for the body. Even though the response is usually beneficial, often life-saving, for the body, in most cases it brings aftereffects. These consequences of the immune response are together called inflammation.

Since ancient Roman times, medical professionals have recognized the major signs of inflammation, even though the complexities of the immune process were not clear to them. The four cardinal signs of inflammation are:

Redness – caused by dilated blood vessels
Heat – caused by increased blood flow
Swelling – caused by extra blood and fluid flowing into the affected area
Pain – caused by swelling and by certain chemicals released by immune cells during their response

This description has stood the test of time, and most of us immediately recognize these sensations from times when we’ve been injured or have fought off a cold virus, scratched an insect bite, or suffered from allergies.

Example of the immune response when our skin is injured

The immune response can occur for many different reasons, as a reaction—sometimes an overreaction—to many different stimuli, and for varying periods of time. But the mechanics of the immune response and the inflammatory consequences are similar no matter the circumstances. The inflammatory hallmarks of redness, heat, swelling, and pain are predictable.

See the diagram for a representation of a typical immune response—in this case, an injury to the skin—and the four signs of the inflammatory consequences.

The most common inflammation-related conditions

Immune response and consequent inflammation occur in many different circumstances. Below I list the most common conditions that involve immune system activity.

Injury

When we get a scratch, cut, fracture, broken bone, or other injury, immune cells work to catch and kill foreign invaders or toxins, dispose of dead tissue and debris, and start the healing process. This immune response usually results in inflammation in and around the injured area: swelling, redness, heat, and/or pain.

Infection (bacterial or viral)

After exposure to a virus or bacteria like a cold or flu virus, immune cells mobilize and attack the invaders, with consequent inflammation in the form of a swollen sore throat, muscle aches, and congestion. Some infections can become chronic, like untreated Lyme’s Disease, and in the worst cases, the immune system is unable to contain the infection.

Allergies

In some people, the immune system can mistake otherwise harmless substances like pollen, peanuts, milk, or cat dander to be dangerous invaders. The immune cells mount a response, and the sufferer feels the inflammation consequences—redness, swelling, pain—until the immune attack subsides or is countered with medication.

Autoimmune diseases

When the immune system attacks the body’s own tissues, mistaking them for dangerous substances, we call this an autoimmune condition. We still don’t know enough about whether genetics, infection, toxins, or something else causes most cases of autoimmune disorders, but the inflammation that comes from constant immune system reactions can be very painful and destructive. Some of the most common autoimmune diseases are rheumatoid arthritis, Lupus, Crohn’s disease, multiple sclerosis, ulcerative colitis, Graves’ disease, Hashimoto’s thyroiditis, and celiac disease.

Other inflammatory-related diseases

When the immune system is having trouble clearing out invaders in a particular body organ or body part, or is triggered in some other way, there can be severe localized inflammation as the immune cells and proteins attempt to gain the upper hand. Inflammatory diseases of this kind usually include the suffix “-itis,” such as tonsillitis, laryngitis, bronchitis, and appendicitis. There are dozens of “-itis” diseases, too numerous to name here, and all are characterized by inflammatory consequences—swelling, redness, heat, and/or pain.

Low-grade internal inflammation (also known as “chronic inflammation”)

Immune cells can wage fights on the cholesterol that lines people’s coronary arteries, on compounds released from excess body fat, on unstable molecules in the blood or tissues, and on compounds that enter the bloodstream from our diet. These fights can result in a type of constant, internal, low-grade inflammation in our blood, tissues, and organs that we might not be able to feel, but can have harmful effects on our health. We’ll be discussing this type of chronic inflammation at length in future posts and will not go into further detail here.

Conclusion

We all experience the immune response and consequent inflammation practically every day. Some of us are unlucky enough to have to deal with dire consequences of immune system overreaction and debilitating inflammation. Medications, avoidance of allergens, and special diets can help some of the most serious types of conditions covered above. And for low-grade, internal inflammation—aka, chronic inflammation—we can choose to adopt a diet that can help lessen immune cell reactions and excess inflammation.

What is Inflammation?