The relationship between exercise and immune function follows a J-shaped curve. Sedentary individuals have above-average infection risk. Moderately active individuals have below-average infection risk, significantly below the sedentary baseline. Very high-volume athletes, particularly endurance athletes in heavy training blocks, have infection risk that rises back above the sedentary baseline. This J-curve is one of the most consistent findings in exercise immunology and has significant practical implications for how to structure physical activity to support rather than suppress immunity.
How Moderate Exercise Enhances Immunity
Each bout of moderate exercise mobilises immune cells from lymphoid tissues and the bone marrow into circulation. Natural killer cells, T cells, B cells, and neutrophils all increase in number during exercise and migrate preferentially to potential sites of infection and tumour activity. This immune surveillance function is enhanced by regular moderate exercise over months, producing durable increases in circulating immune cell counts and improved immune cell function. A meta-analysis of exercise and upper respiratory tract infection found that 150 minutes per week of moderate exercise reduced infection incidence by 33 percent compared to sedentary controls.
The Open Window After Intense Exercise
After a bout of high-intensity or prolonged exercise, there is a period of three to 72 hours during which immune function is transiently suppressed. During this open window, circulating immunoglobulins fall, natural killer cell activity decreases, and mucosal IgA in the saliva drops significantly, reducing mucosal immune defences. This is the window in which elite athletes are most susceptible to respiratory infections, and it is most pronounced after races, high-intensity competitions, or training blocks of unusually high volume. Managing recovery nutrition, sleep, and stress during this window is critical.
Practical Immune-Supportive Exercise
The immune-optimal exercise pattern is moderate intensity, 60 to 75 percent of maximum heart rate, for 30 to 60 minutes, five or more days per week. Resistance training produces similar immune benefits to aerobic exercise with less pronounced post-exercise open window suppression. For adults managing immune conditions, autoimmune disease, or recovering from illness, moderate exercise is clearly beneficial. Returning to high-intensity training too quickly after infection prolongs immune recovery and increases reinfection risk.
- Target 150 minutes per week of moderate-intensity exercise for optimal immune benefit
- After unusually intense exercise, increase sleep, reduce social exposure, and prioritise nutrition in the following 24 to 72 hours
- Resistance training twice weekly provides immune benefits with lower post-exercise suppression risk than high-volume endurance training
- Do not exercise intensely when symptomatic: above-the-neck symptoms are the traditional threshold for light exercise, below-the-neck symptoms require rest
- Track resting heart rate and heart rate variability: sustained elevation indicates immune-relevant training stress
Sources
- Nieman, D.C., & Wentz, L.M. (2019). The compelling link between physical activity and the body's defense system. Journal of Sport and Health Science, 8(3), 201-217.
- Walsh, N.P., et al. (2011). Position statement: part one and two on immune function and exercise. Exercise Immunology Review, 17, 6-63.