Scientists at Israel’s Technion Institute have stimulated the feel-good center in mice and discovered that their immune cells were able to kill twice as much bacteria than those of non-stimulated mice.  The results, published in nature medicine, could explain the placebo effect.

http://www.timesofisrael.com/israeli-study-shows-placebos-physically-boost-immunity/

http://www.nature.com/nm/journal/vaop/ncurrent/nm.4133/metrics/news