For your body, a sudden and drastic change in temperature constitutes an attack – as anyone who’s ever fallen overboard in British waters will concur. And, whilst “attacking” your own body may not sound like a good thing, there is no harm in keeping it on its toes. In fact, quite the opposite.  Scientists from the Czech Republic immersed witting subjects in cold water for one hour, three times a week and monitored their physiology. They found significant increases in white blood cell counts and several other factors relating to the immune system. This was attributed to the cold water being a mild stressor which activates the immune system and gives it some practice.