The immune system is challenged by environmental events such as trauma, psychological stress and ageing. As with most health concerns, diet and exercise are the two areas you should ensure are well managed by you. Adopting a healthy life-style is your best defence against environmental attacks on your immune system. What we already know: don't smoke, don't drink to excess, keep active, exercise very regularly, get enough sleep, eat plently of fruit and vegetables,avoid stress where possible, maintain good hygiene such as washing your hands, and take care in cooking such items as meat thoroughly.
To date, we don't know whether you can actually boost your immune system because which cells would you choose? Taking supplements does not appear to be the answer, and the evidence from those who stay healthy into their nineties strongly suggests the healthy life-style is all-important. So don't be a slouch or a couch potato for starters.