The mechanism of effect of prostaglandins and catecholamines on NK cells has been elucidated (1): beta- adrenergic receptors and prostanoid receptors on the cell membrane cause elevated levels of intra-cellular cAMP when activated by their ligand, which in turn interferes with the NK cell cytotoxic activity against cancer cells or viral pathogens. Since the elevated intra-cellular levels of cAMP is caused by each type of receptor independently, blockade of only one of these pathways will enable an increase in intracellular cAMP levels via the other pathway, and have no effect of the suppression of cytotoxic activity. Therefore, simultaneous blockade of both pathways may be more effective. Research in rats directly supports this hypothesis (7).