Relaxation and immunity enhancement effects of gamma-aminobutyric acid (GABA) administration in humans 

This two-part human clinical study from 2006 aimed to demonstrate the change in brain waves from anxious beta waves to more relaxed and alert alpha waves after supplementation with GABA in 13 healthy subjects (21-35 yo.) The second portion of the study examined the impact on acute stress induced immune suppression in eight healthy volunteers. The GABA treatment showed the most significant increase in alpha waves (and concurrent decrease in beta waves). L-theanine group also showed an increase in alpha waves though not as much as the GABA group alone.  

References:

Abdou AM, Higashiguchi S, Horie K, Kim M, Hatta H, Yokogoshi H. Relaxation and immunity enhancement effects of gamma-aminobutyric acid (GABA) administration in humans. Biofactors. 2006;26(3):201-208. doi:10.1002/biof.5520260305 https://pubmed.ncbi.nlm.nih.gov/16971751/