PESSIMISM PRODUCES DEPRESSION. And when a person is depressed, certain brain hormones become depleted, creating a chain of biochemical events that end up slowing down the activity of the immune system. For example, two key players in our immune systems are T cells and NK cells.

T CELLS recognize invaders (like viruses) and make more copies of themselves to kill off the invaders. Pessimists’ T cells don’t multiply as quickly as optimists’, allowing invaders to get the upper hand.

NK CELLS circulate in the blood and kill whatever they come across that they identify as foreign (like cancer cells). Pessimists’ NK cells can identify foreign entities, but they don’t destroy them as well as the optimists’ NK cells.