Breathing is essential to our survival and to our good health. We can live more than 50 days without food and about 7 days without water. But, without oxygen we cannot survive more than about 5 minutes. In many cultures, breath (qi, chi, prana) is considered the vital link to energy, awareness, composure, and ultimately to transcendence.
There is a new body of research that shows that all of the breathing disorders, from asthma to tuberculosis, from emphysema to interstitial lung diseases, have been linked with an overactive immune system. The immune system is not only system to benefit from proper breathing. The brain benefits as well. Research from Japan shows that during relaxed abdominal breathing, brain waves also show a pattern of relaxation. Finally, heart rate variability (HRV) has been studied extensively. Poor HRV has been linked with increased mortality after heart attack, and has also been shown to be linked with depression, anger, and anxiety. Research has found that proper breathing can improve HRV and reduce immune activation.
The bottom line: when we consciously and mindfully focus on our breathing, a rhythmic pattern of healthy heart rate variability and healthy immune function result. And that mean a longer and healthier life. For cancer patients, you can also use proper breathing to reduce pain.
Dr. Liponis  describes, there are three types of breathing:
The second breathing pattern is the most common kind. Your chest and lungs will be expanding, but the expansion is restricted by tension and tightness in the muscles around the abdomen and ribs. This causes the chest to expand mainly upward, with less airflow and more rapid respiration.
The third kind of breath comes from the abdomen and uses diaphragm. When the diaphragm contracts, your lungs expand, pulling air in through your mouth like bellows. When you breathe from your abdomen, your belly will expand and move out with each inhalation. Your chest will rise slightly, but not nearly as much as with chest breathing; your abdomen is doing all the moving.
Doing abdominal breathing, you can activate vagus nerve and trigger a relaxation response. The relaxation response, which is the opposite of the stress response, is necessary for your body to heal, repair, and renew.
sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).
Acetylcholine is responsible for learning and memory. It is also calming and relaxing, which is used by vagus nerve to send messages of peace and relaxation throughout your body. New research has found that acetylcholine is a major brake on inflammation in the body . In other words, stimulating your vagus nerve sends acetylcholine throughout your body, not only relaxing you but also turning down the fires of inflammation which is related to the negative effects from stress.
Exciting new research has also linked the vagus nerve to improved neurogenesis, increased BDNF output (brain-derived neurotrophic factor is like super fertilizer for your brain cells) and repair of brain tissue, and to actual regeneration throughout the body. For example, Theise et al.  have found that stems cells are directly connected to the vagus nerve. Activating the vagus nerve can stimulate stem cells to produce new cells and repair and rebuild your own organs.
There are many ways to activate the vagus nerve and turn on the relaxation response. When you take a deep breath and relax and expand your diaphragm, your vagus system is stimulated, you instantly turn on the parasympathetic nervous system, your cortisol levels are reduced, and your brain heals.
To practice deep breathing, inhale through your nose and exhale through your mouth. Remember to:
You can proceed as follows: take a breath into your belly (i.e. expanding your diaphragm) to the count of five, pause for a second, then breathe out slowly through a small hole in your mouth. While at rest most people take about 10 to 14 breaths per minute. Ideally, reduce your breathing to 5 to 7 times per minute. Exhaling through your mouth instead of nose makes your breathing a conscious process, not a subconscious one.
As you do this, your muscles will relax, dropping your worries and anxieties. The oxygen supply to your body's cells increases and this helps produce endorphins, the body's feel-good hormones. Tibetan monks have been practicing this to modulate the effects of stress for decades. They don't practice these ancient techniques to improve their memory, fight depression, lower blood pressure, or heart rate, or boost their immune systems, although all of those happen.