Exercise can mobilize the body's natural killer cells, which seek
out and destroy cancer cells and cells infected by viruses. An increased
oxygen uptake from the blood can also neutralize free radicals. Excercise
also reduces stress which can deplete the immune system. Choose an exercise
that you can enjoy. Swimming cycling, walking, Tai Chi, Yoga whatever.
Isometrics, where you oppose one muscle against another or against an imovable object, don't help your cardiovascular system. This type of exercise raises blood pressure and could be harmfull to people with heart trouble.With general exercise, you pump more blood with only a little more pressure. With isometrics, you don't pump more volume, so the pressure is elevated.
Any exercise should be preceeded with proper warm up exercises that focus on the muscle groups that will be used. Warming up muscles reduces injuries. One should also cool down the muscles with similar stretching exercises to reduce soreness later on.