e it soththing rious toto think aboutut. uh, nonot really lookoking forward d to it. ( yelllls ) to activate the response i'm hopi to see, i need to o do somethingngis streressful. oh, that's's cold! not jujust physicalllly stress, but mentallyly. the aim to indu a t of s strs, but not actually get hurt. ooh! ( man laughing ( water gurgling ) and just o once. michael: immunolologist drdr. nalie ririddl, from university llllege london, takekes generous a amounts of my y blood. right, you'll probably ve t to pu quite a bit of pressure on thehere. - i willll, yeah. whirriring ) michael: while i u up, natalie trtracts the imimmune cells from my blooood samples. the cells that mobize duri the stress s response tend to have a more, kind of, aged chacteristicic so, generally, as people get older you see more of tse age cel---- you haveerery few of thehese "aged celells." - hurray!! - anand seem to hahave quite a young g imne systete - a young immune system. a young g immu systetem. - and that'sood,d, is it? - thisis is good. ( chuckleses ) ok, so whahat happened?? so here, we'e're looking a at cs caed a a natural kilillecell and ththese cells arare known