Studies of this include all aspects of the human body a brief description of the influence of laughter on them are:
Heart It has been shown that laughing helps protect the heart. Although studies are not sure why laughing protects the heart, the studies do explain that mental stress impairs the endothelium, which is the protective barrier
lining a person's blood vessels. Once the endothelium is impaired, it can cause a series of inflammatory reactions that lead to cholesterol build up in a person's coronary arteries, which can ultimately cause a heart attack. Also according to Sultanoff in his interview for the article for WebMD, laughter has been show to increase tolerance of pain and boost the body's production of infection-fighting antibodies, which can help prevent hardening of the arteries and subsequent conditions caused thereby such as angina, heart attacks, or strokes. It is shown that people who are "chronically angry and hostile have a greater likelihood for heart attack, people who "live in anxious, stressed out lifestyles have greater blockages of their coronary arteries", and people who are "chronically depressed have a two times greater change of heart disease. According to a 2006 article on WebMD, studies at the University of Maryland found that when a group of people were shown a comedy, after the screening their blood vessels performed normally, whereas when they watched a drama, after the screening their blood vessels tended to tense up and restricted the blood flow.
Diabetes A study in Japan shows that laughter lowers blood sugar after a meal. Keiko Hayashi, Ph.D., R.N, of the University of Tsukuba in Ibaraki, Japan, and his team performed a study of 19 people with type 2 diabetes. They collected the patients' blood before and two hours after a meal. The patients attending a boring 40 minute lecture after dinner on the first night of the study. On the second night, the patients attend a 40 minute comedy show. The patients' blood sugar went up after the comedy show, but much less that it did after the lecture. The study found that even when patients without diabetes did the same testing, a similar result was found. Scientists conclude that laughter is good for people with diabetes. They suggest that chemical messengers made during laughter may help the body compensate for the disease.
Anxiety & Children
According to an article of WebMD, studies have shown that children who have a clown present prior to surgery along with their parents and medical staff had less anxiety than children who just had their parents and medical staff present. High levels of anxiety prior to surgery leads to a higher risk of complications following surgeries in children. According to researchers, about 60% of children suffer from anxiety before surgery.
The study involved 40 children ages 5 to 12 who were about to have minor surgery. Half had a clown present in addition to their parents and medical staff, the other half only had their parents and medical staff present. The results of the study showed that the children who had a clown present had significantly less pre-surgery anxiety.WebMD 2005
All of this information has been found on wikipedia.ord as well as WebMD.