While the reasons for immune system involvement in the regulation of fat tissue remain unclear, Dr. Artis says immune control of fat tissue is likely a conserved evolutionary response to body stressors. "For example, imagine that a person infected with the flu is sick, feverish and has no appetite. One can envision the value in reprogramming metabolic processes — such as holding on to stores of body fat — to help the body get through this stressful period associated with infection.