Yes, exercise does boost your immune system. But like anything, too much of a good thing could prove costly.
Scientific studies show that too much exercise actually weakens the immune system. That's why Neil Walsh and his team at Bangor University set out to find to what extent is working out actually harmful to the immune system. Unlike previous studies on the subject, Walsh and his team measured immune function "in vivo" rather than test tubes or animals. For the study, the scientists administered a chemical called diphenylcyclopropenone (DPCP) through a patch on the lower back, 20 minutes after exercise stints of various lengths and intensities. The researchers compared 30 minutes of "moderate" treadmill running, 30 minutes of "intense" running, 120 minutes of "moderate" running, and a control group with no running.
What they found was that a long duration of exercise causes a temporary weakening of the immune system, while short intense workouts do not. With that being said, the experiment does leave a lot of questions unanswered. For example, would long bike rides, which are generally less "stressful" on the body compared to long runs, generate the same effect? Is 120 moderate minutes simply a harder workout than 30 intense minutes? While the answers to these questions remain to be seen, the study is a step in the right direction in understanding how duration and intensity affect immune function.