For the most part, exercise is beneficial for your health, including the health of your immune system. But if you have ended up sick after a particularly long or strenuous workout, you know that exercise can also negatively affect your health. Whether an exercise program strengthens or weakens your immune system depends on the length, frequency and intensity of your workouts.
The Immune System
The immune system includes both adaptive and innate divisions and consists of numerous types of cells, most of which are varieties of white blood cells, or leukocytes. B cells and T cells are responsible for adaptive immunity. They respond to microbial invasions by generating antibodies so you can more effectively fight off future infections. The innate immune system serves as a first line of defense, protecting you from new as well as old pathogens. Cells of the innate system include natural killer cells, neutrophils and macrophages.
Short-Term Effects of Exercise
A bout of exercise affects the cells of the immune system in varying ways, depending on the duration and intensity of exercise. Typically, immune system activity increases during exercise. It may stay elevated, continue to increase or decline in the recovery period following. Moderate exercise results in an increase in natural killer cell activity, while a long bout of high-intensity exercise can reduce neutrophil and natural killer cell function, as well as B cell and T cell activity.
Longer-Term Effects of Exercise
Regular moderate aerobic exercise strengthens the immune system. It results in improved natural killer cell numbers and activity, and forestalls declines in T cell function that occur with age. However, more prolonged exhaustive aerobic training acts as a stress on the body and can lead to declines in natural killer cell activity, perhaps due to the influence of the stress hormone cortisol, which is released during exercise.
Overtraining and Weakening of the Immune System
Because an extreme exercise regimen acts as a stress on the body, it is important to allow time for proper recovery. With sufficient recovery, your body adapts to the stress by becoming more fit. However, if you rush back into training too soon, you run the risk of overtraining, which can lead to a chronically depressed immune system. In fact, one of the signs of overtraining, along with fatigue and declining athletic performance, is frequent upper respiratory tract infections such as colds and sore throats.
References
- "Exercise Physiology for Health, Fitness and Performance"; Sharon A. Plowman and Denise L. Smith; 2011
- "Exercise Physiology: Energy, Nutrition, & Human Performance"; William D. McArdle, et al.; 2007
