The true determinants of our immune system's strength are within our control: diet, fitness, rest and emotional wellbeing are the keys to bolstering your immunity.; the power to positively impact immunity begins with a better knowledge of how it works. A healthy, vital immune system depends on the proper function of every other bodily system. As humans largely come into contact with bacteria and viruses via the mucus membranes of the nose, mouth and gastrointestinal tract, our ability to ward off illness relies heavily on the health of the respiratory and digestive systems.