And guess what?
These important NK cells actually respond to your mood.
The most important emotion to boost these NK cells is laughter.
When you are feeling good, optimistic and happy, your NK cells multiply exponentially.
If you are known pessimist, there is still hope for you.
A study done at the University of California showed that actors who spent time performing depressing movie scenes had a decreased immune responsiveness. On the other hand, actors who performed roles where they were uplifting had an increased immune responsiveness. So the moral of the story is something called Fake it till you make it. Yes, even if you don't feel happy, at least act like you do! Your body's immune system will reward you for it.
Here is a good punch list to improve your mood and of course boost your NK cells.