This module explores the gut–immune system connection, highlighting the gut as a major immune organ and its role in maintaining immune tolerance, protecting against pathogens, and preventing disease. It covers gut anatomy, Gut-Associated Lymphoid Tissue (GALT), and key immune cells such as T-cells, B-cells, dendritic cells, and macrophages. The module also discusses essential nutrients that support gut barrier integrity and immune function, including vitamins, minerals, omega-3 fatty acids, and polyphenols. It further examines dysbiosis, intestinal permeability (“leaky gut”), their contributing factors, and their potential association with autoimmune, allergic, metabolic, and neuropsychological conditions.