Being fit is good for the healthy longevity of components and systems within your body. Via the open journal Immunity & Aging, here is a look at fitness and the aging immune system: "Highly conditioned elderly men seem to have relatively better preserved immune system than the sedentary elderly men. Long-term endurance training has the potential to decelerate the age-related decline in immune function ... elderly runners showed a significantly higher T cell proliferative response and IL-2 production than sedentary elderly controls. IL-2 production was similar to that in young adults. Their serum IL-6 levels [a pro-inflammatory cytokine] were significantly lower than their sedentary peers. ... Of the various components of the immune system, T cells are the most sensitive to the effects of aging. Mitogen-induced T cell proliferation is usually reduced, and this may be the result from disruption of the well-balanced network of regulatory cytokines."