Diet plays an essential role in immune function. Consequently, deficiencies or insufficiencies in vitamins or minerals can impair immune function. The gut microbiota also modulates immune function, so maintaining a healthy gut is crucial. However, when the immune system becomes overactive, autoimmune disorders can arise. A growing body of evidence indicates that the fasting-mimicking diet may be beneficial in treating autoimmune disorders. In these Aliquot segments, Dr. Rhonda Patrick and Dr. Valter Longo describe the roles that diet and fasting play in maintaining or restoring proper immune function.