It may be easy to assume that since exercise is beneficial to every other component of the human body, it must be beneficial to the immune system as well. However, studies have shown that the effects of exercise on the immune system are variable. Results depend on the intensity and duration of an exercise bout. Other factors which are essential to these studies include the type of exercise performed and the frequency of performance.

A link has been established between physical exercise and cancer incidence. The susceptibility for several types of cancer can be reduced with moderate, consistent exercise training. These cancers include colon and reproductive cancers of the breast, prostate, ovary, uterus, cervix, and vagina. With increased exercise, levels of reproductive hormones produced and circulated in the blood are decreased. This altered state decreases the risk for cancer.(URL, 3) Based on this fact, it has been determined that non-athletes are at a two and a half times greater risk for these cancers. Recommended activity consistsof moderate intensity exercise, performed for thirty minutes per bout, five days per week. Under this recommendation, the risk of developing reproductive cancer is reduced through the enhancement of immune function.(URL, 3)

The state of the immune system determines susceptibility to and recovery from infections. This fact has led to several studies researching the effects of exercise and tumor incidence with regard to circulating levels of immune cells. One study suggests that with chronic moderate treadmill running there are increases in the number and activity of phagocytic cells, which would indicate a greater level of ingestion and breakdown of foreign material in the body. Eighty five percent of this increase is accountable to macrophages which also perform phagocytosis. However, while there was an effect on circulating levels of phagocytes, tumor weight remained consistent in mice studied over a two week exercise period. This may indicate that circulating levels have an impact on new antigens entering the blood stream, but no effect on the previously established tumor.(Journal, 5) This study also found that heavy or exhaustive exercise may actually inhibit natural killer cell (NK cell) activity during the recovery phase following exercise.

A correlation has also been established between exercise and respiratory tract infections. Moderate levels of exercise appear to decrease the risk for upper respiratory tract infections (URTIs) while sedentary and high intensity athletes are more likely to experience URTIs. During marathon training, the risk for URTIs increased in women as mileage training increased.(Journal, 1) This intense level of exercise caused an immune deficiency which left the athletes more susceptible to infection. This may explain why elite athletes are often unable to compete due to illness or viral infection.(Journal, 2)

To determine the impact of exercise on the immune system, blood analysis is necessary at resting levels or directly following an exercise bout. Levels of T lymphocytes, macrophages, and NK cells are measured and compared to cellular resting levels. Comparisons may also be made between resting or exercising levels over a period of time. Directly following high intensity exercise, the numbers of circulating NK cells increase by 150-300%. However, within one to two hours following exercise (a recovery period) NK cell levels may fall 25-35% below resting levels for a short period of time. Contrasting this to moderate types of exercise, an increase is seen immediately following the exercise bout, however, NK cell levels return to normal without experiencing the depressed state. Continued moderate training over a period of time results in an increase in NK cell activity and number, even in a resting state. This may be due to changes in epinephrine levels during exercise.(Journal, 1)

One study which contrasted previously stated information discussed how NK cells maintained elevated levels through the training season of elite endurance athletes. Regardless of other immune cells, circulating NK cells were more prevalent in endurance athletes as opposed to sedentary individuals or moderately trained athletes. These elevated levels decrease during off season performance.(Book, 4) Also, immediately following high intensity exercise, an increase in T lymphocytes and other leukocytes is seen. Levels of these cells returned to normal within thirty minutes of exercise completion. This study also described increased levels of monocytes and macrophages with exercise, however, after several days of high intensity exercise, macrophages were less likely to enter cavities in mice where inflammation occurred. This may indicate, even though elevated levels of leukocytes were present, cells were not performing at maximum capacity. It is possible that this phenomenon occurs with other cells of the immune system as well.

Exercise has beneficial effects on the physical body and also the mental state. Patients who exercise claim to have a more positive mental attitude and an improved quality of life.(Journal, 1) Along these lines, stress, both mental and physical, has been shown to have physiological effects on the immune system through changes in hormone levels. Exercise tends to regulate levels of circulating hormones, thereby decreasing stress.(Journal, 4) In some instances, stress is beneficial to the system. It stimulates a release of epinephrine (also referred to as adrenaline) which causes leukocytes to be released from lymphoid organs. This reaction is extremely important when individuals are injured or preparing for surgery. The elevated levels of leukocytes aid in recovery

Although unrelated to exercise, high fevers (body temperatures reaching 104 degrees Fahrenheit) kill potential cancer cells within the body. These susceptible cells cannot endure the heat. This dangerous fever may actually cleanse the body of certain antigens allowing the immune cells to focus on and over come the disease at hand

Finally, exercise has an effect on food intake. An elevated number of calories are being utilized by the body which must be replaced through an elevated caloric intake. Proper food combining is essential to adequately replenish sources and prepare the body for future exercise. Nutritional recommendations can be found in the nutrition page linked from the introductory page.

With all the information and studies presented here, it is evident that the field has massive potential for development. An advance could solidify information and delve deeper into the physiological aspects which take place in the immune system as a result of exercise. All in all, the research suggests that moderate levels of exercise enhance the immune system, while heavy levels or a lack of exercise may cause reduced immune system potential.