The interrelationship of the HPG-thymic axis and immune system regulation.

It is well established that gonadal steroids play an important role in the regulation of immune response. These substances exert their effect through estradiol and possibly dihydrotestosterone receptors located in the thymic reticuloepithelial cell matrix. In vivo treatment of male rats with estradiol has been shown to depress the cell mediated immune response as mediated through thymic serum factors. Castration stimulates the cell mediated immune response while thymectomy inhibits this effect. It also appears that growth hormone at physiological concentrations can directly inhibit thymocyte blast transformation in vitro while leutinizing hormone, follicle stimulating hormone, and prolactin do not directly affect thymocyte function. A tentative scheme for immune system regulation by the HPG-thymic axis is presented.