After drinking for seven months, the macaques received another booster shot, and their reactions were remarkably different. The immune systems of the bad monkeys that drank too much failed to produce the antibodies the body usually makes in response to a vaccine. The moderate drinkers, on the other hand, demonstrated an enhanced immune response—better even than the teetotaling control group. The researchers can't yet fully explain the results, but one possible explanation is that modest amounts of alcohol stimulate the immune system.