[L]aughter reduces levels of certain stress hormones. In doing this, laughter provides a safety valve that shuts off the flow of stress hormones and the fight-or-flight compounds that swing into action in our bodies when we experience stress, anger or hostility. These stress hormones suppress the immune system, increase the number of blood platelets (which can cause obstructions in arteries) and raise blood pressure. When we're laughing, natural killer cells that destroy tumors and viruses increase, as do Gamma-interferon (a disease-fighting protein), T-cells, which are a major part of the immune response, and B-cells, which make disease-destroying antibodies.
(Can you tell it's Friday?)