It is obviously important to address the health of your immune system. Two of the best ways to do this are (1) through yoga and exercise and (2) through what you eat. Most people don’t realise that these two things are intimately related. How you practice yoga, exercise and especially how you breathe directly affects your choice of food.
Yoga has postures and breath-control (pranayama) exercises that stimulate the immune system. One such posture is Simhasana (see photos), which directly stimulates the spleen meridian. In the two photos shown of Simhasana you see me doing Tha-Uddiyana Bandha on exhalation retention. This bandha stimulates both the spleen and the thymus gland which are both important organs of the immune system. In the second Simhasana photo I am doing Tha-Uddiyana Bandha on exhalation retention retention with Tha-Mula Bandha (AKA nauli). This is a very powerful combination that acts in many ways on the immune system. These exercises are also powerful on the immune system because with regular practice they cause an overall increase in carbon dioxide which leads to an increase in circulation as well as an increase in the relaxation response (the parasympathetic nervous system).
The ability to relax the abdomen and especially the sides of the abdomen reflects a regular practice of abdominal or diaphragmatic breathing. Diaphragmatic breathing directly stimulate the immune system. One of the best places to learn diaphragmatic breathing is any of the variations of mayurasana (as shown in the third photo). In the mayurasana you see me doing I have maintained a very soft abdomen yet that is exactly where I am breathing from. The pressure from my elbows and my diaphragm on inhalation directly stimulates the internal body systems, especially the immune system, the digestive system and the reproductive system.
You don’t need to learn difficult postures to get this effect. A similar effect that can be just as positive on the immune system by doing a brisk but relaxed one hour walk every day, but the trick is you have to move your hips and shoulders freely with the spine and you must breathe into your abdomen. In this situation the spine massages the internal organs and the abdominal breathing stimulates the parasympathetic nervous system and gives reflex relaxation of the muscles around the lower trunk thus freeing up the spine. As excess trunk muscle tension is relieved with diaphragmatic breathing and the circulation of blood can also increase significantly without breath rate or even heart rate increasing.
When minimal and diaphragmatic breathing is combined with postures that physically put pressure on the organs and/or acupuncture meridians related to the immune system and the digestive system, then the combination not only enhances immune system function but also increases your digestive function. Hence food becomes more easily assimilated, and elimination of waste is more effective. Therefore you can eat less because food will satisfy you more easily.
The effect of learning how to breathe less than normal (hypoventilation) creates a mild internal acidity that makes one tend to choose food that is more alkalising in the blood. Alkalising foods include most fruit vegetables and salad, namely all the foods that have been shown to positively affect the health of the immune system.
Hence, how you practice yoga does not simply affect the health of your internal organs but actually changes your desire for food. As you get more comfortable creating a mild respiratory acidity by breathing less air per minute (the ultimate aim of pranayama), you begin to tend to choose healthier more alkaline food, and not crave less healthy acidic foods which are often processed and can negatively affect the immune system and your health in general.