As far as I know there is no empirically documented evidence at the moment as to how regular lifestyle Budo training may affect the immune system. This of course would vary from art to art as well since, for example, getting your head pummeled a lot is not helpful to the brain-stem.
However I’ve notice some very interesting things over the years of Budo, particularly serious Aikido training. Powers of recovery are stimulated, attitudes which lead to recovery are vivified, minor injuries forces the body to continually act in a manner of a living being as opposed to a sedentary one. Whilst it may be argued that all physical activities lead to improved health, and no one could possibly disagree with this, Budo also includes dealing with threat, even if only imagined. This causes the body’s response mechanisms to dig deeper than that of a more dormant personality.
Even light training. Even simply overcoming the inertia of turning up. All this simulates the dutiful attendance of the protective powers of the universe whether in the form of a warrior or a T- lymphocyte and other protectors of life.
Most sensible budoka who train regularly not only show better powers of recovery, improved health but also longevity. Even the ones who misuse their bodies with bad diets and other unhealthy habits.
Anything which is properly maintained not only performs better, but also lasts longer. So also the human body-mind. The patterns ingrained in the genetic DNA not only reflect the spiral movements of the universe but also of billions of years of cellular survival. Protozoa soon learned that by joining together to make forms they could sustain a biological ecosystem and the lessons, information gathering, etc, longer and better by grouping and specializing within that group such and in an organic biological entity.
Ancient Vedic scriptures consider existence in such depth as we are only beginning to rediscover their scientific basis. That, er, “sixth day” of “creation” long identified in ages past, lasted quite a few million years. The ancient Aryavedantins called those “days,” “Days of Brahma. In case you may be interested that’s merely 1000 mahayugas, which is equal to 4.32 billion years. Whew! But a blink of an eye from “up above.” Interestingly physicists are daily discovering these “primitive” superstitions to be more correctly based than more recent ones. By the way, the term Brahma is the name they give to the Creative principle of the Supreme, Light and Expansion=Bri (Sanskrit)=Hikari (Japanese). They also acknowledge the destroyer/recycler principle named Shiva and the preserver/architect/blueprint/heart principle in the name Vishnu. Seven of these Days of Brahma make a solar manvantara taking our tiny earth globe as an instance, at the end of seven such cosmic “days” the seven subplanes of immense time frames almost beyond comprehension to the mind of a three score and ten denizen. The original stuff not only reads better and makes more sense scientifically, but also is not watered down for kindy mentalities. It also does not, for example, reduce the Supreme to the level an anthropomorphized tyrant, as have tribally superstitious lost minds. Further, these texts concur with current and new scientific discoveries. But that’s another study.
As for the immune system, movement is eternal in the cosmos and on earthy movement means life. Stagnation is death. Harmonious movement re-enacts the movements of nature and the great universe. Harmonious response to discord empowers most terribly.
Individually, our immune system is of immensely vital importance.
What then is the collective immune system of society and global humanity?
And, what condition is it in right now?
The question is this: In the scheme of things what part does your life play?
Are you part of the problem, or part of the solution? Are you a warrior who proactively protects the greater good, or a coward who buries his head where he can’t see the injustices all around him? Or are you an exploiter? Or a mere drifter, hiding where it’s most comfortable?
Does your life form part of the T-cells or the infections of life on this earth?
Do you add to conflict or to harmony?