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The relationship between exercise and and immunity

Q: How does regular physical activity support a normal immune function?

Q: What is the relationship between exercise and immunity?

Krishna:

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Regular, moderate physical activity supports a normal immune function by improving immune cell circulation, reducing chronic inflammation, and enhancing the body's ability to detect pathogens. (1)

Acute exercise is an immune system adjuvant that improves defense activity and metabolic health. Data support a clear inverse relationship between moderate exercise training and illness risk. Exercise training has an anti-inflammatory influence mediated through multiple pathways.

Increased carbohydrate and polyphenol intake is an effective nutritional strategy for immune support. Habitual exercise improves immune regulation, delaying the onset of age-related dysfunction.

How does exercise support immunity?

These are the pathways: (1)

Better Circulation of Immune Cells: Physical movement pumps blood and lymph fluid through your body. This flow recruits white blood cells, natural killer cells, and T-cells from the spleen and lymph nodes into the bloodstream, helping them patrol for and catch intruders more efficiently.

Exercise causes changes in antibodies and white blood cells (WBCs). WBCs are the body's immune system cells that fight disease. These antibodies or WBCs circulate more rapidly, so they could detect illnesses earlier than they might have before. However, whether these changes help prevent infections is another question still remains to be authenticated.

Lower Chronic Inflammation: Regular workouts help reduce systemic low-grade inflammation, which allows your immune system to respond without being overworked or distracted by chronic inflammatory damage.
Cellular Readiness: Studies show that regular exercisers have immune cells with higher gene readiness, meaning reservist cells and threat-detecting cells are primed to react faster when exposed to bacteria or viruses.
Stress Hormone Regulation: Consistent activity helps balance stress hormones like cortisol, preventing long-term hormone elevations that can otherwise weaken immune defenses.

Temperature controlled infection: The brief rise in body temperature during and right after exercise may prevent bacteria from growing. This temperature rise may help the body fight infection better. This is similar to what happens when you have a fever.

However, the scenario changes in athletes during periods of intensified training and competition. Overdoing exercise alters immunity by triggering a 'temporary' "open window" of immune depression caused by surging stress hormones, cellular redistribution, and high physical stress without enough recovery.

Please note that this is not a permanent one.

For competitors taking part in endurance sports, exercise causes immune cells to change in two ways(2).

Initially, during exercise, the number of some immune cells in the bloodstream can increase dramatically by up to 10 times, especially ‘natural killer cells’ which deal with infections. After exercise, some cells in the bloodstream decrease substantially – sometimes falling to levels lower than before exercise started, and this can last for several hours.

Many people previously interpreted this fall in immune cells after exercise to be immune-suppression. However strong evidence suggests that this does not mean that cells have been ‘lost’ or ‘destroyed’, but rather that they move to other sites in the body that are more likely to become infected, such as the lungs.

Scientists know that these cells are not ‘destroyed’ for three main reasons.

First, most evidence shows that cells return to normal levels within several hours, which is far too quick for them to be ‘replaced’ with new cells.

Second, studies in humans have shown that these cells have the ability to leave the bloodstream and travel to other body sites.

Third, studies with laboratory animals have shown by labelling immune cells, that following exercise, these labelled cells accumulate in the lungs, and other places, because they go there to look for infections.

Researchers, therefore, suggest that low numbers of immune cells in the bloodstream in the hours after exercise, far from being a sign of immune-suppression, are in fact a signal that these cells, primed by exercise, are working in other parts of the body.(2)

Key Reasons for this temporary Immune Suppression during heavy exercises

Stress Hormone Surge: Intense workouts lasting longer than 90 minutes cause the body to release high levels of cortisol and adrenaline. These hormones suppress certain immune cell functions to manage immediate physical trauma.
Immune Cell Redistribution: Hard training temporarily lowers the number of circulating lymphocytes (like natural killer cells and T cells) in the bloodstream. Many of these cells move to tissues and potential infection sites like the lungs, leaving the blood 'temporarily' depleted.
Lower Salivary IgA: Prolonged exertion decreases the levels of secretory immunoglobulin A (sIgA) in saliva, which acts as a primary defense line against upper respiratory tract infections.
Lack of Recovery: Overtraining syndrome occurs when repeated intense workouts happen without proper rest, sleep, or nutrition. This keeps the body in a chronic state of physical stress and systemic inflammation, impairing white blood cell production and function.

Moderate-intensity exercise—such as a 20-to-30-minute brisk walk, cycling, or swimming most days of the week—provides the ideal boost to immune surveillance.

Don't overdo it. Because extremely intense, long-duration workouts (like multi-hour marathon training without proper rest or nutrition) can cause a 'temporary' dip in immune function, leaving the body briefly more vulnerable to infections if you are not trained properly.

Although a strenuous exercise by itself will not increase the likelihood of catching an infection, other factors might. First, attending any event where there is a large gathering of people, increases your chance of infection. Second, public transport, particularly airline travel over long distances, where sleep is disrupted, may also increase your infection risk. Other factors, like eating an inadequate diet, getting cold and wet, and psychological stress, have all been linked to a greater chance of developing infections.

Footnotes and sources:

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC6523821/

2. https://www.bath.ac.uk/announcements/research-debunks-myth-that-str...

3. Best TM, Asplund CA. Exercise physiology. In: Miller MD, Thompson SR. eds. DeLee, Drez, & Miller's Orthopaedic Sports Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 6.

4. Jiang NM, Abalos KC, Petri WA. Infectious diseases in the athlete. In: Miller MD, Thompson SR. eds. DeLee, Drez, & Miller's Orthopaedic Sports Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 17.

The relationship between exercise and and immunity

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