Human gammadeltaT lymphocytes are a subset of T cells (white blood cells) and are a first line of defense against microbes and tumors.
According to these researchers L-Theanine was found to enhance gammadelta T cell function in individuals supplemented with this amino acid (Bukowski JF, Percival SS, 2008). This decreased the incidence of cold and flu symptoms in the study participants.
l-theanine promotes nitric oxide production in endothelial cells. The endothelium is the inner layer of the blood vessels and nitric oxide improves circulation by producing vasodilation (opening of the blood vessels).
l-theanine administration in a laboratory study resulted in enhanced nitic oxide production and thereby vasodilation in the artery (Siamwala JH, et al., 2013).
Bukowski JF, Percival SS. L-theanine intervention enhances human gammadeltaT lymphocyte function. Nutr Rev. 2008 Feb;66(2):96-102.
Jamila H Siamwala, Paul M Dias, Syamantak Majumder, Manoj K Joshi, Vilas P Sinkar, Gautam Banerjee, Suvro Chatterjee. L-theanine promotes nitric oxide production in endothelial cells through eNOS phosphorylation, J Nutr Biochem. 2013 Mar;24(3):595-605.