This review suggests that some fraction of immune system failure in later life can be laid at the feet of a sedentary lifestyle over the years - and that starting to exercise when older is not going to reverse all of that decline. "The elderly population is at an unprecedented risk of infectious diseases and [cancer] due to apparently inevitable age-related declines in immunity. The 'immune risk profile' (IRP) is an array of biomarkers that has been used to predict morbidity and mortality in older adults. As it is generally accepted that middle-aged and elderly individuals who habitually participate in moderate-intensity exercise are less likely to incur an infection than their sedentary counterparts, this review addresses current knowledge on the effects of regular exercise on aspects of adaptive immunity as they relate to the IRP. Findings from cross-sectional studies mostly show enhanced immunity in physically active compared to sedentary older adults. These include greater T-cell responsiveness to mitogens in vitro, a reduced frequency of antigen-experienced and senescent [T-cells], enhanced IL-2 production and T-lymphocyte expression of the IL-2 receptor, longer chromosome telomere lengths in blood leukocytes and in vivo immune responses to vaccines and recall antigens. In contrast, the evidence from the available longitudinal studies that have used an exercise training intervention in previously sedentary elderly to improve similar immune responses is less compelling."