Theanine is a compund found in tea and can be extracted from tea leaves. Theanine is often sold as a dietary supplement in the United States. Able to cross the blood-brain barrier, theanine has psychoactive properties and has been shown to reduce mental and physical stress. Theanine improves mental capacity and mood when taken with caffeine. While structurally similar to the excitatory neurotransmitter glutamate, theanine has only a weak affinity for the glutamate receptor. Theanine may help the body increase immunity by increasing T-Cell counts. Drinking lots of tea has similar affects as taking the Theanine supplement.