In this study, conducted at the Indiana State University Sycamore Nursing Center,  33 healthy adult women were divided into two groups. The treatment group watched a humorous video, while the control group viewed a tourism video. All participants completed questionnaires regarding their stress and humor levels before and after watching their videos. In addition, blood drawn before and after treatment was tested for natural killer cell levels.

Compared to the control group, the laughter group reported a significant decrease in stress following treatment. Their stress level appeared inversely correlated with their level of mirthful Laughter. Participants with high scores on the humor questionnaire also had significantly higher numbers of natural killer cells after treatment.

The laughing participants?

Natural killer cell levels were significantly higher than those of the control group too.

Research has looked at both humoral (immunoglobulins) and cellular immunity. In the case of the former, most of the studies have focused on immunoglobulin A (IgA). IgA resides in the mucosal areas and helps protect you against upper respiratory infections. Seven studies have shown significant increases in concentrations of IgA in response to comedy programs designed to produce a lot of laughter.
While many different investigators have completed the IgA research, Lee Berk and his associates at the School of Medicine at Loma Linda University has obtained similar findings for many additional components of the immune system.
Mirthful laughter also increases levels of IgM and IgG. IgM antibodies are the first to arrive at a location within the body as a part of the humoral immune response. After IgM does its initial work, IgG takes over. It is IgG antibodies that are produced in the greatest amount in the body, and that are responsible for long-term immunity. When you are immunized, for example, it is the IgG antibodies that are tested to see if the procedure was successful.

Laughter also increases levels of Complement 3, a part of your immune system that helps antibodies pierce through defective or infected cells in order to destroy them.

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