B-cells are a type of white blood cell that play an important role in the immune system, and some of these cells secrete a protein known as IL-35, which has been shown to regulate inflammation. Because the microorganisms living in the digestive system can have critical effects on the immune system of their host, scientists recently set out to uncover the link between these microbes and IL-35 production. The team found that certain microbes inhabiting the guts of mice, such as Lactobacillus bacteria, can promote the generation of IL-35-secreting B-cells and that they do this by producing 3-indoleacetic acid (IAA) in the presence of lipopolysaccharides. They also found that IL-35 may help prevent mice fed a high-fat diet from becoming obese and observed lower levels of IAA in obese mice than in nonobese mice. While these findings have been demonstrated only in mice so far, they might help elucidate how gut microbes affect IL-35 secretion and obesity and could help pave the way for the development of new therapies to combat autoimmune disorders and other chronic diseases.