In yoga the chest-opening poses are the most effective immunity boosters. Intriguingly, these poses most people will "cheat"-- in the sense they will have flawed execution. It is important for the teacher to constantly check how these poses are executed because of the potential to slump at the chest.
Some people, not used to yogic stretches or who have weak breathing habits, may even feel dizzy when trying them in the first few weeks. So, in these cases they have to be phased and eased into the body, with props, if essential.
Interestingly, these poses not only work on the spot between the shoulder-blades, but also work on the chest region, impacting the thymus gland powerfully. Thymus gland produces the T-cells, our immune cells. By early adulthood, its functions begin to wobble if not activated, as is done, through yoga. As we have seen, even exercise (other than yoga) can put a heavy load on the immune cells. The liver steps in to take some of load off such toxic pile-up that occurs daily and routinely. Yogic twists, as wells as forward bends, work on the liver, massaging it and toning it back to youthful vigor so it complements the work of the thymus gland effectively.
Practices like the surya namaskar, or dynamic practices (called the flow or vinyasa or druta or dynamic variations) clean up our lymphatic system(lymph vessels, lymph nodes), which is also involved with immunity, through the production of lymphocytes, a type of immune cells and filtering of antigens. Poses which apply pressure and resistance (such as the locust, all prone poses) also boost bone health. Bones becomes healthily dense when muscles around them work-out powerfully, as happens with yogic poses. Then the bones, especially the long ones which are actually cell-producing factories, become more efficient at manufacturing immune cells such as B-cells, phagocytes.
The nascent science of psycho neuro-immunology tells us that negative mind-sets, such as depression, anxiety, fear, anger, are implicated in a weakened immunity. While modern medicine has just woken up to the inevitable connection between mind and body, our ancient yogis knew it all along. So, not surprisingly, all immune-boosting yogic poses and practices are also mood uplifting. Nothing is left to chance, but all care is taken to ensure that the health acquired through yoga is not superficial but a thorough one which seeps the entire personality.