TH2 sensitization in the skin-gut-brain axis: How early-life Th2-mediated inflammation may negatively perpetuate developmental and psychologic abnormalities.

We recently reported children with comorbid atopic dermatitis (AD), asthma, allergic rhinitis, and food allergies displaying a 2.7-fold increase in developmental delays. To this end, we hypothesize unregulated increases in T helper-2 (Th2)-driven inflammation, such as those seen in atopic diseases, can exert deleterious effects on the developing brain. Recognizing that available information is incomplete and that many potential associations are not firmly established, we speculate these effects underlie the association between Th2 sensitization and cognitive dysfunction in children. In this review, we explore the role of Th2 sensitization in the skin-gut-brain axis and explain how it can lead to reduced connectivity and transmission in the developing brain. With a focus on AD, we explore the association between Th2 sensitization and developmental abnormalities such as developmental delays, memory impairment, autism spectrum disorder (ASD), and epilepsy/seizures. As such, we review the available literature to examine the impact of increased IL-4 exposure in early life on the brain. We explore the possible association between Th2 sensitization and psychologic dysfunction such as attention-deficit/hyperactivity disorder (ADHD), depression, anxiety, and suicidal ideation. We also examine the impact that increased exposure to glucocorticoids and neurotrophins in early life exerts on the developing brain. Last, we discuss future directions for the advancement of our knowledge as a scientific community including possible interventions to reduce developmental and psychologic aberrations in children.