Stress-responsive hormones, including but not limited to adrenal corticosteroids and catecholamines, have a myriad of effects on various aspects of the immune response in both down- and upregulatory fashion.(23) Often hormones affect immunity in an 'inverted U-shaped' way, being suppressive at either abnormally high or abnormally low levels. As mentioned above, the hypothalamic-pituitary-adrenal axis, itself triggered by immunological as well as psychological events, is immunoregulatory, and both corticotrophin-releasing factor and ACTH have direct effects on immunity in addition to those via induction of release of cortisol. Growth hormone increases T- and NK-cell functions in aged animals. Prolactin antagonizes glucocorticoid-induced immune suppression. Gonadal hormones affect immunity. NK cell activity is higher in the luteal phase of the menstrual cycle (which should influence surgeons in the timing of cancer surgery in premenopausal women). Cellular immunity is depressed during pregnancy. Thyroid hormones may stimulate NK activity and affect T-cell development, and may modulate the affect of cytokines on immune cells/24)
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