When it comes to figuring out the biological causes for obesity, theories abound: is the problem hormonal? The result of exposure to a toxin? Purely genetic? A mistake of evolution? Now we can add one more possible explanation to the pile: immunity. A new study found that not only can ILC2s, a type of immune cell, defend the body against infection and allergens, but they also may protect it against fat. Will we one day be taking immune boosters instead of dieting? The research is just getting started.