In comparison with controls, patients with ADHD have elevated concentrations of pro-inflammatory cytokines (such as IL-6 and TNF-α) and reduced levels of anti-inflammatory cytokines (IL-4, IL-2, and IFN-γ) and brain-derived neurotrophic factor (BDNF) [31,32,38]. This evidence concerns the gene IFNG and attention deficit-hyperactivity disorder.