“Laughter increases the number and activity of some white blood cells called ‘T’ and ‘B’ cells, natural killers that fight viral infections and some types of cancer cells. It multiplies the antibody IgA, which fights upper respiratory tract infections. A hearty chuckle builds up gamma interferon, which stimulates the various components of the immune system.
“Laughing speeds up our heart rate, and some have likened this to ‘internal jogging.’ It’s an aerobic activity that works the diaphragm and increases the body’s ability to use oxygen. That’s why after a big laugh you often feel the need to sigh and take a big breath of air. It sure beats thirty minutes on the treadmill.
“As we laughed, our blood pressure temporarily is elevated; however, that is followed by a prolonged, mild decrease in blood pressure. Laughter also lowers various hormone levels—the kind associated with the fight-or-flight response—and makes us feel less stressed, more relaxed. That’s why people say things like ‘I laughed so hard I couldn’t get up’ or ‘I laughed so hard I fell over.’