Want to give your kids an immune system boost? Try letting them play in the dirt more often, a new study suggests. Researchers in Finland found that when they brought nature into daycare playgrounds — including forest soil and vegetation — preschoolers’ immune function showed a change for the better. In simple terms, it shifted to a less inflammatory state. That immune system redirect was also accompanied by some changes in the children’s microbiome — the vast collection of bacteria and other microbes that naturally live on and in the body. Research has revealed those bugs to be vital in normal body processes — from metabolism to brain function to immune system regulation. It’s too early to know whether bringing the forest to urban playgrounds has real health benefits, experts said.