IL6 and attention deficit-hyperactivity disorder: Proinflammatory serum cytokines (proteins serving as signaling molecules within the immune system) released in AD, such as interleukin (IL-)4, IL-6, IL-13, or TNF-α, can cross the blood–brain barrier and potentially interfere with the maturation of prefrontal cortex regions and neurotransmitter systems involved in the pathophysiology of ADHD [39,40,41,42,43].