As you might know, one of the roles of the immune system is to hunt down and destroy cancerous cells before they can grow into tumors. That the immune system becomes damaged and dysfunctional in later life is one of the many reasons why cancer risk increases with age. Here is a paper that shows that exercise boosts the capacity of immune cells to kill cancer - as though you needed another reason to regularly exercise, given its many benefits to healthy longevity. "Natural killer cells (NK) induce the death of tumor cells [by] cytotoxicity, whereas platelets reduce the capacity of NK to destroy tumor cells. Physical exercise affects both immune function and platelet activity because responses depend on type, intensity, and duration of exercise. This investigation explores how various exercise regimens influence platelet-impeded cytotoxicity of NK to [tumor cells] ... Thirty sedentary men performed on three occasions moderate exercise), severe exercise and severe exercise after warm-up exercise ... Severe exercise enhances the cytotoxicity of NK to [tumor cells] and simultaneously promotes the platelet-impeded apoptosis of [tumor cells] induced by NK. However, warm-up exercise reduces the resistance of platelet to NK-[tumor cell] interaction, increasing the efficiency [of] cytotoxicity by NK after severe exercise."

Link: http://www.ncbi.nlm.nih.gov/pubmed/19092699