Both exercise and happiness lead to increased production of antibodies, which are a special type of protein produced by the immune system. When everything is working normally, antibodies arrive on the scene shortly after antigens -- bacteria, viruses or other unwanted invaders -- are detected in the body. Antibodies hunt them down and bind themselves to the antigens. Once locked on, T-cells -- the immune system's "killer cells" -- arrive and destroy the antigens.
Antibodies stick around in the body after they're created in case the particular antigen they were designed to hunt shows back up. Antibodies also help produce other cells that aid and assist in the immune system.
People who are happy are more resistant to diseases ranging from the common cold to heart disease, while stress and anxiety tend to make people more susceptible to sickness, including diabetes and stroke [source: Cohen]. Happiness has been shown to increase antibodies by as much as 50 percent [source: Shimoff].
Exercise is no slouch, either -- it has been shown to increase antibody production by as much as 300 percent [source: Nieman]. Exercise also directly increases the number of T-cells in your body. By increasing numbers of "hunters" (antibodies) and "killers" (T-cells), it's no wonder a workout keeps you healthier. And since people who exercise tend to report higher levels of happiness, the exercise-induced happiness improves the immune system independent of the physiological effects of the exercise.
One of those effects is lowering cortisol levels in the body, a hormone that's released when your body experiences stress. A little bit of it helps the immune system, but too much weakens your ability to fight illness. The fact that exercise decreases the level of cortisol in your body may also explain why it decreases feelings of anxiety.
There can be too much of a good thing, however. While exercise bolsters the human immune system overall, intense exercise actually weakens it in the recovery period that follows the activity [source: Kapasi]. This is not an uncommon occurrence with elite athletes, and seniors especially should keep in mind that intensive exercise may weaken their resistance to disease.
Studies have shown that meditation increases antibody production and, simultaneously, feelings of happiness [source: WebMD]. This may in part be due to meditation's similarities to what we traditionally perceive as being exercise, namely breath control, focus, and the tuning out of external stimuli.