The Biology of Inflammation

The Biology of Inflammation

An illustration of a neutrophil and red blood cells.

Hi, I’m Angie. Today, I’m returning to my role as a science educator and sharing information with you about the biological processes behind chronic inflammation.

Our Immune System
In order to better understand the biology of inflammation, we need to understand some basic information about how our immune system works. Our environment has many organisms capable of causing disease. These organisms are called pathogens and include bacteria, protists, fungi, and viruses. A pathogen lives and reproduces best within a host, the organism the pathogen invades. We are constantly exposed to pathogens in our food and water, on the surfaces we come in contact with, and in the air we breathe. We become hosts to some of these organisms.

The human immune system evolved for protection from such pathogens. It is constantly searching our bodies for signs of disease-causing microorganisms. When found, our immune system first identifies the nature of the pathogen, then it sends specialized cells and chemicals to fight the infection. Finally, when the infection is gone, the immune response stops so it won’t cause any unnecessary damage to our healthy cells.

Our Immune Response
Our bodies’ immune responses can be either innate or adaptive. Both are important in helping us maintain our health. The adaptive immune response is what we often think about when we hear the word immunity. The adaptive response comes from stored information about past infections. Our bodies quickly recognize a pathogen and initiate a more rapid response when reexposed.

Knowledge of our adaptive immune response led to the development of vaccines. Vaccines reduce the chance of infection from a particular disease by helping our body recognize a pathogen quickly and fight it off before it overwhelms us- all without ever having had the disease.

While the adaptive immune response provides a more specific line of defense, the innate immune response is our first line of defense against infection. It works very fast, but it is non-specific and has no memory. Because of this, it isn’t strong enough to tackle an infection on its own. It simply helps to reduce the workload for our adaptive immune response.

While our skin, tears, and mucus serve as barriers to many pathogens, those that get through are met by specialized cells and chemicals. Macrophages gobble up pathogens, natural killer (NK) cells destroy virus-infected cells, cytokines act as chemical messengers, and mast cells release histamines.

The Good, the Bad, and the Icky
Yes, those kinds of histamines! The same chemicals that cause the annoying symptoms associated with allergies and colds are one of our body’s best innate defenses against infectious diseases.

Our body’s normal immune response begins with recognizing a pathogen. Once recognized, chemical messengers called cytokines signal other cells that the pathogen is present and needs to be destroyed along with any infected cells.

The second step in a normal immune response is the triggering of an acute inflammatory response. The cytokines mentioned above are considered pro-inflammatory, meaning they encourage inflammation to occur. Many people think the localized redness, swelling, heat, and pain that may occur is caused directly by an infection or injury. It is actually our body’s response to try and contain an infection and support healing.

Remember the mast cells I mentioned earlier? They are an important part of the inflammatory response by releasing histamines along with other chemicals such as heparin. The histamines bind with cells that make up the walls of blood vessels causing them to contract. This contraction causes the vessels to leak which allows blood from the vessels to seep into surrounding tissues (permeability). At the same time, the histamines stimulate chemicals that cause these same cells to relax causing the vessels to dilate. The dilation allows more blood to move through the vessels. Last but not least, heparin acts as an anti-coagulant preventing the blood from clotting. What does all of this mean? More blood gets directly into the affected tissues. Why is this important?

This blood carries with it two important types of cells involved in wound healing and defense against pathogens- neutrophils and monocytes. Neutrophils are the first responders to infection or trauma. They play a key role in our innate immune response. They can move quickly and easily to the infection site where they engulf and digest pathogens. They can also release toxins that kill or inhibit bacteria and fungi and send signals to recruit other immune cells to the site of infection. Monocytes are capable of developing into macrophages. Macrophages consume pathogens and cells damaged by pathogens.

If all of these things are happening to help rid us of infection and increase healing, why do we feel so icky? Maybe this chart will help explain things:

How Might This Affect Me?
As I’ve already stated, inflammation is an important part of the immune system’s response to injury and infection. Without it, wounds wouldn’t heal, and infections could become deadly. But what if the inflammatory process continues beyond healing and fighting off infection? What if inflammation occurs where it is not needed?

As we’ve evolved, changes in human DNA have made it more difficult for certain pathogens to thrive and infect us. Humans who had the DNA mutations that selected for an inflammatory immune response were healthier and lived longer more productive (and reproductive) lives.

Our patients recovering from shoulder surgery or rehabbing a sprained ankle actually benefit from the acute inflammation that also produces redness, swelling, pain, and temporary loss of function.

Unfortunately, some people have highly sensitized immune systems that overreact to substances in the environment. As a result, specialized cells and chemicals are summoned to battle an infection or help heal an injury but have nothing to do and nowhere to go. They end up fighting the very body they came to protect. This overreaction produces a low level of inflammation throughout the body. We call this chronic inflammation. Common symptoms of chronic inflammation include fatigue, joint pain, depression and/or anxiety, diarrhea and/or constipation, and weight gain or loss. It often has no symptoms at all.

Because chronic inflammation has been linked to heart disease, stroke, cancer, and certain autoimmune disorders such as Crohn’s disease, Lupus, and IBS, it is important to maintain a healthy lifestyle to keep inflammation in check. Reducing inflammation can also help reduce joint pain.

Things you can do to reduce inflammation in your body are:

  • Eat lots of anti-inflammatory foods. (Like our Cedar Hill Physical Therapy & Wellness Center facebook page to find weekly Mediterranean diet recipes.)
  • Cut back or eliminate inflammatory foods such as sugar and HFCS, artificial trans fats like margarine, refined carbohydrates, excessive alcohol, and processed foods including processed meats.
  • Make time to exercise. Motion is lotion for the joints! Make time for 30 to 45 minutes of aerobic exercises 4 to 5 times per week.
  • Increase muscle strength through weight or resistance training.
  • Manage chronic stress through meditation, yoga, guided imagery, or getting out in nature.
  • Make an appointment to see Paul the Physical Therapist for specialized exercises to help you build strength and reduce inflammation.


Domínguez-Andrés, Jorge, and Mihai G. Netea. “Impact of Historic Migrations and Evolutionary Processes on Human Immunity.” Trends in Immunology, vol. 40, 12, Dec. 2019, p. 1105–1119.

Szalay , Jessie. “What Is Inflammation?” LiveScience, 19 Oct. 2018, https://www.livescience.com/52344 inflammation.html.

Watson, Laura J. “Immune Response.” Geeky Medics, 20 Feb. 2021, https://geekymedics.com/immune-response/.

Zedalis, Julianne, and John Eggebrecht. “Biology for AP® Courses.” OpenStax, 8 Mar. 2018, https://openstax.org/books/biology-ap-courses/pages/1-introduction.