People only have a small number of immune cells circulating around the body. Those cells prefer to hang out in lymphoid tissues and organs like the spleen, where your body kills viruses, bacteria, and other microorganisms that cause disease.
Because exercise increases blood and lymph flow as your muscles contract, it also increases the circulation of immune cells, making them roam the body at a higher rate and at higher numbers.
While you do get an immediate response from your immune system when you exercise, that will eventually go away—unless, that is, you keep working out consistently.
Another benefit of exercise is that it decreases inflammation in the body—which, in turn, can also improve immunity.
Having a compromised immune system is a matter of frustration for older people as their muscles shrink with age and their immunity declines. Fortunately, resistance training can prevent this from happening.
A regular exercise program is associated with a lower risk of some cancers, in part because it maintains the quality of our muscles, keeping them lean.