beta 2-Adrenergic receptors, which are encoded on chromosome 5q32-34, belong to the group of G-protein-linked hormone receptors with seven transmembrane domains. Upon agonist binding, adenylate cyclase is activated. Although the function of human leukocyte beta-adrenergic receptors is unknown, these cells are often used as a model system to study tissue beta-adrenergic receptors. In intact mononuclear leukocytes or membrane preparations, 1000-3000 beta 2-adrenergic receptors are found, having an antagonist affinity constant (KD) in the range of 25 pM. beta-Adrenergic receptor numbers are different in leukocyte subsets, with receptor density higher in B than in T cells. CD56+ or CD57+ natural killer cells express more receptors than CD8+ or CD4+ cells. KD is higher in CD8+ than in CD4+ cells. Acute sympathetic activation by isoproterenol infusion or short-lasting exercise leads to an increased number of mononuclear beta-adrenergic receptors with a slightly reduced proportion of those with high agonist affinity. Acute sympathetic activation by adrenaline infusion, short-term exercise, or psychological stress also causes a selective increase in circulating CD56+ or CD57+ lymphocytes which are rich in beta-adrenergic receptors. The results of several studies suggest that adrenaline-induced changes in beta-adrenergic receptors and the redistribution of leukocyte subsets may be linked. beta-Adrenergic receptors may mediate immuno-modulatory effects by causing selective cell mobilization.