I knew about autophagy, a Japanese scientist received Nobel prize for it. I think there is some kind of misunderstanding with terms diet here. I think you meant starving yourself, not dieting. Fasting would not reduced your immune system. A protein called Sirtuin act by repairing our DNA when we're not eating. That's the basis of Hunger make you healthy. I think you should check out Brain Derived Neurotrophic Factor (BDNF) as well, it will gives you a valuable insight on fasting and ketogenic diet.