From this question, I learn ways of the boosting immune system, but i didn't find main word laughing, so does laughing improve immune system?
There is some evidence that laughter can help improve eustress levels and overall health, but evidence is mixed as many of the studies are not designed very well.
This study examined stress hormones in a small sample study of men viewing a comedy video, and concluded that stress hormones dropped in the subjects which has a beneficial effect in the body.
Many of the studies available, however, are in the nature of this example, which is observational in nature, and may or may not have correlation. (This citation is abstract only, I do not have access to the full article on this).
So while there is evidence that laughter and good feelings may help boost personal health, it is very much unproven and in need of further, well designed research.