['Tumor cells can reduce the number of dendritic cells (DCs) in the tumor environment and cause DC dysfunction through autocrine or paracrine pathways. We sought to measure cyclooxygenase-2 (COX-2) expression in bombesin-inhibited DCs treated with theanine in vitro and to explore the protection and activation effects of theanine on DCs.', 'Enzyme-linked immunosorbent assay (ELISA), reverse transcription-polymerase chain reaction (RT-PCR), and Western blotting were used to analyze the effects of theanine on COX-2 expression and interleukin (IL)-12/IL-10 secretion of bombesin-treated DCs.', 'DCs acquired an impaired phenotype as a result of bombesin treatment. Theanine increased the expression of mature DC surface molecules. The number of cell apoptosis with the treatment of bombesin and theanine significantly decreased, accounting for 15.9%, compared with 26.1% of cell apoptosis with bombesin. COX-2 expression in bombesin-treated DCs was inhibited by theanine in a dose-dependent manner. Theanine promoted DC secretion of IL-12. IL-12 levels reached (137.4 ± 4.9) pg/ml with theanine at 200 µmol/L. However, theanine inhibited the secretion of IL-10 in a dose-dependent manner. IL-10 levels were only (58.4 ± 6.9) pg/ml with theanine at 200 µmol/L.'] Does theanine improve the function of dendritic cells via the downregulation of cyclooxygenase-2 expression?