Strengthening of the Immune System
Our immune system is obviously crucial to sustaining good health. We are constantly bombarded by viruses, bacteria and other antigens that could seriously harm or even kill us. This threat is present every day of our lives, so anything we can do to bolster the effectiveness of this system is clearly important. But can a good sense of humor really make your immune system stronger?
It has long been recognized that chronic stress (including the negative emotion that accompanies stress) weakens the immune system, leaving you more vulnerable to illness - although short-term, acute stress may boost the immune system.12 If you find yourself constantly stressed out by the demands of nursing, a financial crisis, deteriorating health, an unsatisfactory marriage or any other persisting personal problem, your odds of coming down with some kind of health problem increase. But can positive emotion - and specifically the positive emotion associated with humor - somehow support good health? If it does, immunoenhancement would be one obvious place to look for an explanation of how humor contributes to health.
By the early 1980s, researchers finally began to study the impact of humor and laughter on the immune system. The best evidence that humor boosts the immune system comes from studies where immune system measures are taken before and after a particular humorous event - usually a comedy video. But research showing that individuals with a better sense of humor have a stronger immune system is also important, since it shows the importance (for your health) of making the effort to improve your sense of humor.
Immunoglobulin A
The greatest amount of research to date (35-40 studies) has focused on immunoglobulin A, a part of your immune system which serves to protect you against upper respiratory problems, like colds and the flu. Secretory IgA (or SIgA) is a kind of antibody found in the mucosal areas of the body - including saliva. It is the first line of defense against antigens in these areas. There are now more than a dozen studies showing that watching as little as 30 or 60 minutes of a comedy video is enough to increase both salivary IgA13 and blood levels of IgA.14 This has been shown for both adults and children.15 (It should be noted that a few studies have failed to show this increase, but the general thrust of the data clearly favors an immunoenhancement effect.) These findings should not be taken to mean that you won't get a cold or the flu if you have a good sense of humor. Rather, they suggest that humor and laughter may reduce your odds of catching a cold or the flu - or reduce their severity if you do become sick.
Humor, of course, is just one tool for generating positive emotion. Other approaches to generating a positive emotional state have similarly produced increased SIgA.16 So anything you do to keep yourself in an upbeat, positive mood helps keep this part of your immune system functioning well.
Natural Killer Cells
Along with IgA (a part of your humoral immune system), several different kinds of cells offer a different kind of immune protection. These include natural killer (NK) cells, B and T cells. The most studied among these are NK cells. Several studies have now shown that watching a humorous video increases the activity - and number - of natural killer cells,17 although there is some evidence that this may be true only for individuals whose NK cell activity is lower than average.18 
Natural killer cells have the role of seeking out and destroying tumor cells in the body, as well as battling the latest cold- and flu-generating viruses and other foreign organisms. These cells destroy tumor cells and viruses by releasing a toxic substance. They are another part of the body's first line of defense and can attack foreign organisms even if they've never seen them before.
Among cancer patients, reduced natural killer cell activity is associated with an increased rate of spread of tumors.19 So the significance of laughter's ability to increase the activity of these cells is clear. The previously-mentioned finding that humor's ability to boost NK cell activity is greatest among those with lower levels of NK cell activity is especially important for cancer patients. This is one reason oncology units of hospitals have become so interested in humor as a form of therapy.20
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One especially important study suggested that the level of functioning of one's immune system is very sensitive to shifting moods or emotional states throughout the day. Method actors were asked to generate the emotion of joy within themselves. Amazingly, an increase in the number of NK cells circulating in the blood stream occurred within 20 minutes. Once they got themselves out of this positive state, their levels of NK cells quickly dropped again.21 This kind of finding makes it very clear just how important it is to have tools for sustaining a positive emotional state as much as possible throughout the day.
Other Immunoenhancement Effects
Very little research has been completed on the impact of humor on other parts of the immune system (i.e., other than IgA and NK cells). For example, other immunoglobulins (in addition to IgA) form part of what is called the "humoral" immune response. Only one study (with a small sample of subjects) has been extended to these other immunoglobulins. That study found IgM and IgG to also be enhanced as a result of humor/laughter.22 IgM is the antibody that arrives first as part of the humoral immune response. IgG antibodies are present in the greatest amount in the body and are responsible for producing long-term immunity. When you are immunized for a particular illness, it is the IgG antibodies that are tested to see if the procedure has worked.
This same study showed that watching a comedy video produced increased levels of a substance called complement 3, which helps antibodies pierce through defective or infected cells in order to destroy them.
In terms of other parts of the cellular immune system, again only limited research has been completed, although what has been done supports the findings for NK cells. B cells are produced in the bone marrow and are responsible for making the immunoglobulins. The one study that has looked at B cells found that if you count the number of these cells circulating in the blood before and after a comedy video, you can demonstrate a significant increase in the number of B cells following exposure to humor.23 This is not surprising, of course, given the evidence of increased levels of immunoglobulins following humor.
Another promising study suggests that humor may even have a place in the battle against AIDS. T-cells are another kind of immune cell produced by the thymus gland. The AIDS virus attacks "helper T-cells." Humor and laughter have been shown to increase both the number and level of activation of helper T-cells; they also increase the ratio of helper to suppresser T-cells.24 This is an exciting finding and suggests that a good sense of humor may contribute not only to a patient's ability to cope with the emotional impact of having the disease, but to the body's ability to battle it, as well. Again, however, it should be remembered that we have very little data on humor's impact on these additional components of the immune system.
The humor/T-cell findings are supported by research showing that relaxation techniques increase levels of helper T-cells. For example, medical students' levels of helper T-cells have been shown to be reduced on the day of exams.25 But when half the students were taught relaxation techniques, their level of helper T-cells increased. And the degree of increase was directly related to the extent to which they practiced the techniques learned. So the increased helper T-cell production found for humor and laughter may have been due to the relaxation effect produced by laughter. Consistent with these findings, relaxation techniques have been shown to increase antibody production, NK cell activity, and the effectiveness of cytotoxic T-cells.26
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While we wait for researchers to settle this issue, I fully agree with the advice given by an early long-term AIDS survivor Michael Callen (who died in 1993, before the development of life-extending drugs): "It simply makes sense to try to mobilize whatever immune-system enhancing effects might flow from marshaling the mind. After all, even if your T-cells don't increase, how can having a cheerful, frisky, life-affirming attitude possibly hurt? . . . I highly recommend daily doses of laughter."27
Finally, one study has also shown that humor increases levels of gamma interferon, a complex substance that plays an important role in the maturation of B cells, the growth of cytotoxic T cells and the activation of NK cells.28 It also tells different components of the immune system when to become more active and regulates the level of cooperation between cells of the immune system. Given the specific types of immunoenhancement resulting from humor discussed above, this effect on gamma interferon is to be expected.
Taken as a whole, it's clear that there is something about humor and laughter that causes the immune system to "turn on" metabolically and do more effectively the very thing it is designed to do - promote health and wellness in the face of internal or external threats. But I'm certainly not suggesting that humor is some kind of magic bullet which will cure your patients' cancer or other illnesses. Rather, it creates internal conditions which support the body's own basic healing and health-maintaining mechanisms. It helps assure that these mechanisms are working for you, not against you.
My guess is that future research will show that a major of part humor's power to promote health and healing lies in its capacity to pull us out of the chronic negative mood we're left in by the constant stress in our lives, replacing that mood with a more positive, optimistic outlook that lowers stress hormones and leaves the immune system operating on a higher level. (See section of "The role of mood" below.)
Increased Free Radical Scavenging Capacity
The level of free radicals in the body has received a great deal of attention in recent years, since they have been implicated in faster aging, inflammation, cancer and other pathological conditions. Antioxidant vitamins have become very popular because of their presumed ability to reduce the level of free radicals in the body. It is generally believed that anything which helps reduce free radicals in the body is important when it comes to sustaining health in the long run. Japanese researchers have recently shown that watching a comedy video increases the free radical scavenging capacity in human saliva (this new research is really part of the second wave of health-related humor research, not the first wave - even though it deals with a general health-promoting mechanism).29 Just as importantly, those who reported higher levels of "pleasant feeling" while watching the video showed higher levels of free radical scavenging. This is an exciting finding in support of a key general health-promoting mechanism and is consistent with the bulk of evidence showing that humor boosts the immune system; but more research is needed to document the consistency of this effect.
