The incidence of asthma is increasing steadily. One of the reasons given for this rise is the excessive level of hygiene in our environment. Studies have indeed shown that exposure to a so-called “non-hygienic” environment, rich in microbes, plays a protective role against the development of allergies, including asthma. New research shows that exposure to bacterial DNA drastically amplifies a population of pulmonary macrophages and makes them strongly immunosuppressive, which prevents and treats asthma in mice. This discovery offers promising prospects for the development of a cell therapy based on the administration of these regulatory macrophages to asthmatic patients.