Our immune systems become less active with aging, the number of T cells drop, the B cells also start to wind down, making fewer antibodies that respond to outside intruders and more errant ones that turn on the body itself. UCLA (University of California, Los Angeles) researchers studied whether Tai Chi might re-energize the immune system ^[1]. The study involved 112 healthy people ranging in age from 59 to 86. Half of the volunteers were randomly assigned to attend three, 40-minute tai chi sessions a week. They were taught to practice Tai Chi, The control group attended health education classes that included group discussion. The research participants underwent blood tests for further analysis. The blood tests were done periodically during the study, which lasted a total of 25 weeks. The results were reported in the April 2007 issue of the Journal of the American Geriatrics Society. The T cell activity levels of the tai chi group had risen 38% by the end of the study, compared with a 28% increase for people in the control group. The UCLA researchers noted that Tai Chi alone generated an immune response that was comparable to the vaccine immunity generated by the Shingles vaccine. Tai Chi might have a strong effect on the immune system because it manages to bring exercise, relaxation, and meditation together in "one behavioral intervention," the researchers said. They theorized that Tai Chi enhances T cell activity by quieting the nervous system's “fight or flight” response, which can, in certain circumstances, impacts the immune system.  
[1] Irwin MR, Olmstead R, Oxman MN. Augmenting Immune Responses to Varicella Zoster Virus in Older Adults: A Randomized, Controlled Trial of Tai Chi. J Am Geriatr Soc. 2007;55(4):511-517. doi:10.1111/j.1532-5415.2007.01109.x