The human body has the natural inheritance defence mechanism to fought with external & mental factors to maintain the normal healthy homeostatic state of body & mind. All this is being mainly maintained by the Hormonal & neural control also.
Moreover the immune activity is mainly regulated by the Adrenal Gland. The practice of various mudras like Mahamudra , Uddayan bandhmudra , Shakatichalini mudra , Vipritkarani mudra etc. tone up this Adrenal gland by improving the flow of blood & neural currents for health maintenance of Adrenal & tereby improvement in the immunity against external & mental stresses of life. These mudras also help for proper secretion of these glands & tone up for efficient functioning of these glands