Laughter reduces the level of stress hormones like cortisol, epinephrine (adrenaline), dopamine, and growth hormone. It also increases the level of health-enhancing hormones, like endorphins.
Laughter increases the number of antibody-producing cells we have working for us and enhances the effectiveness of T cells. All this means a stronger immune system, as well as fewer physical effects of stress.
A good belly laugh exercises the diaphragm, contracts the abs, and even works out the shoulders, leaving muscles more relaxed afterward. It even provides a good workout for the heart.
~ via Very Well Mind
How to laugh away stress - Nature
Here is a great article in Nature which talks about laughter producing "mood-elevating hormones called β-endorphins" and "the immunity-enhancing human growth hormone".
They state: "This led the team to wonder whether the link between lowered blood pressure and laughter could be the result of laughter somehow interfering with the production of stress hormones."
The research was by "Lee Berk at Loma Linda University in California".