Quoting from an article about the study:
The researchers say fasting "flips a regenerative switch" which prompts stem cells to create brand new white blood cells, essentially regenerating the entire immune system.
"It gives the 'OK' for stem cells to go ahead and begin proliferating and rebuild the entire system," said Prof Valter Longo, Professor of Gerontology and the Biological Sciences at the University of California.
"And the good news is that the body got rid of the parts of the system that might be damaged or old, the inefficient parts, during the fasting.
In trials humans were asked to regularly fast for between two and four days over a six-month period.
Scientists found that prolonged fasting also reduced the enzyme PKA, which is linked to ageing and a hormone which increases cancer risk and tumour growth.
"We could not predict that prolonged fasting would have such a remarkable effect in promoting stem cell-based regeneration of the hematopoietic system," added Prof Longo.
"When you starve, the system tries to save energy, and one of the things it can do to save energy is to recycle a lot of the immune cells that are not needed, especially those that may be damaged," Dr Longo said.
"What we started noticing in both our human work and animal work is that the white blood cell count goes down with prolonged fasting. Then when you re-feed, the blood cells come back. So we started thinking, well, where does it come from?"
Read the whole article here: Fasting for three days can regenerate entire immune system, study finds.