Weight regain after weight loss is a common problem that can increase the risk of metabolic disease. A new study in Cell Metabolism has identified a population of bone marrow immune cells that can reduce weight regain after weight loss. “We aimed to identify bone marrow immune cell subsets predisposing mice to regain weight,” says first author Hai-Yan Zhou. “Surprisingly, the immune cells in the bone marrow endowed recipient mice with anti-obesity properties, accompanied by increased energy expenditure.”