Childhood allergies and asthma: New insights on environmental exposures and local immunity at the lung barrier

•An intact lung barrier is essential to prevent allergic diseases and/or asthma.•Environmental exposures like air pollution and pathogens impair lung barrier function.•Microbe-rich exposures at farms favor a tight barrier and local immunity.•Airway and gut microbiome can influence immunity and barrier function in the lung.•Diet rich in fibers, indigestible carbohydrates or omega-3/6, influences lung immunity.