There is some evidence both in rats and humans that fasting can have a therapeutic immune cell regenerative effect. Specifically, Multiple Sclerosis is mentioned. During fasting, a certain percentage of immune components are destroyed, and when refeeding occurs post-fasting, the newly generated immune components are disease free.
Source: “Valter Longo, Ph.D. on Fasting-Mimicking Diet & Fasting for Longevity, Cancer & Multiple Sclerosis”, 42 minutes
Link to the study: Diet mimicking fasting promotes regeneration and reduces autoimmunity and multiple sclerosis symptoms