Starting from relevant mechanisms affecting immune regulation, this article reviews the research progress of IL-17 in a selected group of neurological diseases and mental disorders (autism spectrum disorder, Alzheimer’s disease, epilepsy, and depression) from the perspective of neuroinflammation and the microbiota–gut–brain axis, summarizes the commonalities, and provides a prospective outlook of target application in disease treatment. The gene discussed is IL17A; the disease is early-onset autosomal dominant Alzheimer disease.