How can relaxation techniques counteract stress?

Relaxation techniques such as yoga, tai chi chuan, meditation, breathing exercises, and prayer counteract the harmful effects of stress. All of these practices lower blood pressure, produce alpha waves in the brain -- the mental state of relaxation -- and slow the heart rate and breathing. Studies at the University of California School of Medicine in San Francisco found that relaxation techniques even enhance the immune system's ability to fight infection, and that T-lymphocytes (the body's natural "killer" cells that attack invading viruses and bacteria) increase in number after meditation or other relaxation practices.