150 sedentary participants will be randomly assigned to either a 10-month moderate aerobic exercise training program or a sedentary control group. Clinically relevant measures of immune function including the delayed-type hypersensitivity (DTH) response to a battery of antigens and the antibody response to tetanus toxoid and influenza virus vaccination will be assessed before, during and after the intervention. We hypothesize that exercise training will result in improved immune responses including higher peak antibody titers and DTH responses, and sustained levels of protective antibodies.