Exercise and the Immune System

Written By Kyle Rawlek

I used to believe, and you might have heard it too, that doing a lot of aerobic exercise could weaken your immune system. If you push yourself to the extreme, your body can become more vulnerable, that is true. But what I have come to appreciate is that for most of us, when the immune system is doing its job, whether that is after illness, during recovery, or simply for staying well, exercise is not the enemy. It is actually one of our best friends. Especially when it comes to cancer prevention, mild illness recovery, and even helping treatments work better. Moving your body does not just keep you fit, it helps your immune system wake up, stand up, and go to work.

Inside you are cells called natural killer cells. They are the small warrior immune cells whose job is to hunt down and destroy virus infected cells and even tumor cells. Research shows that when you get active, especially with moderate aerobic or strength activity, you can increase how well those natural killer cells function. It is not just how many you have, but how efficiently they go after trouble. And if you can step outside into a green space, a forest, or a park, that adds an extra immune boosting benefit. Time in nature has been shown to raise natural killer cell activity and lower stress hormones like adrenaline, which can otherwise drag your immune function down. You are not just exercising for the sake of being active, you are giving your body one of the best tools it has to help protect you, inside and out.

Exercise and Immune Function Misunderstood for Decades

For many years, people thought that heavy exercise might weaken the immune system. This idea came from studies in the 1980s and 1990s where athletes were asked to record how often they caught a cold or felt sick. Because this was based on self-reporting, the results were not very reliable. Researchers noticed that during endurance races, the athletes’ blood was full of immune cells. Then, a few hours after the event, the number of those cells dropped below the normal level. Scientists at the time assumed that the exercise had destroyed the immune cells, leaving the body open to infection.

In 2018, new research from the University of Bath in the United Kingdom turned that idea around. They tested endurance athletes using saliva samples to detect real signs of infection. They discovered that most of the “illnesses” athletes reported after big races were not true infections at all. When compared with people from the same city who did not race, the athletes were no more likely to be sick. At the same time, animal studies used special dyes to track immune cells as they moved through the body. These showed that after exercise, immune cells do not disappear. Instead, they move to the tissues that need protection the most, such as the lungs, the gut, and even the bone marrow. In those places, they help build new immune cells and strengthen local defenses.

mid.jpg

Immune Signaling

This is called immune signaling. When you move, your muscles release molecules called myokines. These send signals to your immune system to get ready, travel, and repair. Exercise temporarily raises your immune surveillance, meaning your immune cells are circulating, scanning, and responding more actively. When activity ends, the cells move into tissues, where they improve your ability to fight off infection and even recognize and destroy abnormal cells, including cancer cells.

You might wonder why so much of this research involves mice. It is because mice share most of their genes and immune structures with humans, making them excellent models for understanding how exercise affects health. Their studies have helped scientists see exactly how movement triggers these immune responses that protect the body. There is even a monument in Russia dedicated to the laboratory mouse, in gratitude for how much knowledge these tiny animals have helped us gain.

last.png

When You’re Not Feeling Well

So what happens when you come down with something? The good news is that we know quite a bit about how to manage activity when you are sick. The first rule is simple: listen to your body. When you are unwell, your energy is already being used to fight off infection, so lower your exercise intensity. Move in ways that match how you feel, not how you normally exercise. But do remember that some exercise still is good for you and your immune system. 

A few things to note: 

If your symptoms are mostly above the neck—a runny nose, mild sore throat, or headache—then light to moderate activity is usually fine. Keep your pace comfortable, shorten your duration, and stop if your energy drops. Gentle walking, stretching, or slow cycling can even help circulation and keep your immune cells moving.

If your symptoms move below the neck—chest congestion, deep cough, fever, body aches, or stomach upset—then rest is best. Strenuous exercise during those times can delay healing and place extra strain on your immune system. Once your fever and fatigue are gone, and your breathing feels clear, you can slowly start back with light movement. Let your recovery guide you.

Regular activity supports your immune system, but more is not always better. For example, endurance athletes who run more than about forty kilometers a week sometimes see higher rates of upper respiratory illness. For most people, though, moderate exercise a few times a week actually reduces those infections.

How to Safely Get Started and Recover Well

If you are returning to activity after being sick or starting a new routine, begin small and stay consistent. Think of it as training your immune system the same way you would train your muscles.

  • Start with gentle movement. Ten to twenty minutes of walking or light stretching can begin to reactivate your circulation and immune response.

  • Mix it up. Include a blend of moderate aerobic activity (like walking or cycling), some strength work, and an occasional burst of higher intensity once you are feeling strong again.

  • Stay hydrated and nourished. Your immune cells rely on water, nutrients, and rest to do their job.

  • Recover fully. Sleep is where your body repairs both muscle and immune tissues. Prioritize quality rest after activity.

  • Watch your cues. If fatigue or symptoms return, ease off. Recovery days are part of the process, not a sign of weakness.

The best results come from steady, moderate effort—not extremes. Over time, exercise strengthens your immune system, improves heart and bone health, and helps your mind stay sharp and calm. Scientists are still learning all the ways this happens, but there is no question that regular movement keeps your body’s natural defenses stronger and more responsive

Movement as Immune Medicine Booster

Think of movement as one of the simplest and most powerful medicines you have. Every time you move, your blood carries signals that wake up your immune cells and guide them where they are needed most. Regular physical activity helps your body protect itself, clear away damaged cells, and build new ones. Whether you are recovering from illness, rebuilding strength, or simply trying to stay healthy, movement is part of the cure. The more often you choose to move, rest, and then move again, the more resilient your body becomes. Exercise, especially when combined with fresh air and green spaces, is not just about fitness. It is your immune system’s way of remembering how to protect you well.

References:

  1. Gretchen Reynolds. How Strenuous Exercise Affects Our Immune System. (April 2018), https://www.nytimes.com/2018/04/25/well/move/how-strenuous-exercise-affects-our-immune-system.html

  2. Edward R. Laskowsk, MD. Is It Okay to Exercise if I Have a Cold? (February 2017), https://www.mayoclinic.org/healthy-lifestyle/fitness/expert-answers/exercise/faq-20058494

  3. Thomas G. Weidner, PhD, ATC; Thomas L Sevier, MD. (1996) Sport, Exercise, and the Common Cold. National Center for Biotechnology Information (Volume 31), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1318446/pdf/jathtrain00018-0060.pdf

  4. Linda J. Vorvick, MD. Exercise and Immunity (November 2018), https://medlineplus.gov/ency/article/007165.htm

  5. Remy Melina. Why do Medical Researchers Use Mice. (November 2010), https://www.livescience.com/32860-why-do-medical-researchers-use-mice.html

  6. Arana Echarri, A., Martinez Ruiz, R., Ibáñez Lacambra, A., & Vicente-Rodríguez, G. (2023). The effects of exercise training for eight weeks on immune cell characteristics among breast cancer survivors. Frontiers in Sports and Active Living, 5, 1163182. https://doi.org/10.3389/fspor.2023.1163182

  7. Zimmer, G., Haugen, F., & Seiler, S. (2022). Exercise training and natural killer cells in cancer survivors: A systematic review and meta-analysis. Sports Medicine – Open, 8(1), 94. https://doi.org/10.1186/s40798-022-00419-w

  8. Schenk, H. S., Börjesson, M., & Ekblom, B. (2022). The effect of acute physical exercise on natural killer cells: An exercise immunology review. Sports, 10(10), 189. https://doi.org/10.3390/sports10100189

  9. Hanson, K. G., Zschucke, E., & Simpson, R. J. (2024). The effect of exercise and disease status on mobilization of anti-tumour immune cells. Frontiers in Immunology, 15, 1394420. https://doi.org/10.3389/fimmu.2024.139442

  10. Højfeldt, A. J., Pedersen, L., & Pedersen, B. K. (2024). Impact of exercise on cancer: Mechanistic perspectives and new evidence. Frontiers in Immunology, 15, 1474770. https://doi.org/10.3389/fimmu.2024.1474770

  11. Campbell, J. P., Turner, J. E., & Pedersen, B. K. (2024). Mechanistic insight into physical activity pleiotropy in cancer control. Medicine & Science in Sports & Exercise, 56(10), 1932–1941. https://doi.org/10.1249/MSS.0000000000003261

  12. Lee, T. L., O’Leary, A., & Wennerberg, E. (2021). Exercise and the immune system: Taking steps to improve responses to cancer immunotherapy. Journal for ImmunoTherapy of Cancer, 9(7), e001872. https://doi.org/10.1136/jitc-2020-001872

  13. Kim, H. S., Park, J. H., & Kim, Y. H. (2021). Vitamin D and exercise are major determinants of natural killer cell activity. Frontiers in Immunology, 12, 594356. https://doi.org/10.3389/fimmu.2021.594356

  14. González-Gallego, J., Gómez-García, F., & Sánchez-Collado, P. (2023). Immunomodulatory effects of exercise in cancer prevention and treatment. Frontiers in Immunology, 14, 10794543. https://doi.org/10.3389/fimmu.2023.10794543

  15. Durrant, M. R., McCarthy, M., & Wilson, S. M. (2024). Effect of exercise on immune system markers in cancer patients: A systematic review. Journal of Strength and Conditioning Research, 38(4), 645–656. https://doi.org/10.1519/JSC.0000000000004551

  16. Millard, M., Thirion, G., & Le Moigne, A. (2022). Physical activity prevents tumor metastasis through modulation of the immune microenvironment. Frontiers in Pharmacology, 13, 1034129. https://doi.org/10.3389/fphar.2022.1034129

  17. Maroto, J. L., Rojas, V., & Martínez, A. (2024). Acute natural killer cell response to continuous moderate exercise. Current Sports Medicine Reports, 23(5), 204–211. https://doi.org/10.1016/j.csmr.2024.05.006

  18. Turner, J. E., Campbell, J. P., & Simpson, R. J. (2024). Exploring the effects of exercise on immune cell function and tumour outcomes. Frontiers in Immunology, 15, 12159211. https://doi.org/10.3389/fimmu.2024.12159211

  19. Qian, S., Lu, Y., & Xu, X. (2020). Regulation of natural killer cell activity by glucocorticoids, serotonin and exercise. Cellular & Molecular Immunology, 17(10), 1123–1132. https://doi.org/10.1038/s41423-020-0477-9

  20. Woods, P. J., da Silva, G. V., & Gleeson, M. (2020). The effect of acute physical exercise on NK-cell cytolytic activity: A systematic analysis. Sports Medicine, 50(9), 1683–1699. https://doi.org/10.1007/s40279-020-01402-9

Kyle Rawlek
Previous

Fall Awareness
Next

Alternative Sweeteners