As a well-known inhibitory neurotransmitter in the central nervous system, gamma-aminobutyric acid also has critical roles in immune system. Immune cells (e.g., lymphocytes, macrophages) express the components of GABAergic system, including GABA receptors, GABA transporters, and GABA metabolic enzymes. The functions of immune cells are highly impacted on GABA signaling. GABAergic components negatively regulate the immune responses, particularly the T cell-mediated immunity, via their effects on production of pro-inflammatory cytokines and activation of signal pathways, like mitogen-activated protein kinase and nuclear factor-kappaB pathways. These results may indicate that GABAergic components provide a new therapeutic approach for inflammatory and autoimmune diseases, such as experimental autoimmune encephalomyelitis, multiple sclerosis, and inflammatory bowel diseases.