No reputable researcher will claim that exercise will repair a weakened or diseased immune system. Nonetheless, a broad spectrum of studies confirms that moderate exercise can help sustain and strengthen it, even when the effects are indirect.

The role of exercise in helping to lower stress - and the subsequent beneficial effects on health - has been widely studied. Here the studies are less clear, contradicting one another in some details. But overall the conclusion is the same: moderate, regular exercise helps the immune system by moderating the effects of stress.

Most studies carried out over the last 30 years agree: a continual high level of stress has a number harmful effects on overall health. People who experience high stress get more colds, suffer more digestive tract problems and have more frequent bouts of fatigue. Part of the latter is indirect, since it tends to lead to lowered amounts of restful sleep.

Regular exercise helps relieve stress. It does so directly, by providing an outlet for, and consuming much of, the nervous energy produced by stress. It also helps indirectly by shifting one's focus away from the external factors producing the stress.

Exercise can help the cardiovascular system, which in turn improves blood flow, carries away toxins from muscles and organs, and helps keep the kidneys and endocrine system working well. It helps remove germs and circulate antibodies.

All those promote a healthy immune system by lessening the body's susceptibility to disease, while increasing the robustness of the immune system itself.

Exercising increases the body temperature slightly. This, as anyone who has suffered from a cold knows, is the body's natural response to colds, flu and other diseases. The increased temperature helps kill the infecting organisms.

A study at the University of Colorado, Boulder suggests that moderate exercise helps prevent colds as well. It showed that individuals are less likely to get sick after stressful situations when they had engaged in a regular program of moderate exercise. Those that began exercise only on the same day as the stressor didn't enjoy those benefits.

The study was carried out on rats, but one of the reasons those mammals are used is the similarity in some systems, and their responses, to humans.

Exercise programs, undertaken consistently and correctly, help improve body image - that's one of most individuals primary goals in making the effort, after all. That improved body image often leads to higher levels of confidence and relaxation in social situations. That in turn helps reduce stress and enhance the immune system.

Whether the effects are direct or indirect, exercising can help you support and enhance your immune system. That leads directly to better overall health.