Common mold (Aspergillus fumigatus) is one of the most widespread fungal organisms inhaled by people, and can trigger significant allergic responses in asthmatics and patients with Cystic Fibrosis.  Jay K. Kolls, from Louisiana State University Health Sciences Center (Louisiana, USA), and colleagues explored the allergic response by assessing the factors that regulate the body’s sensitization to common mold.  The team found that the protein OX40L was critical in driving the allergic response to A. fumigates, whereby OX40L increased the activity of Th2 cells, and prompted a very strong Th2 response among those patients with the severe allergic response known as Allergic Bronchopulmonary Aspergillosis (ABPA), which occurs in up to 15% of Cystic Fibrosis patients. Further, the researchers observed that immune cells from subjects without ABPA contained higher levels of certain proteins that are essential in the development of allergen tolerance. When the team exposed cells in vitro to vitamin D, they observed a substantial reduction of the protein driving the allergic response, as well as an increased production of the proteins that promote tolerance.