The bacterium Helicobacter pylori is found in the stomachs of half the world’s population. Although persistent infection with H. pylori increases an individual’s risk of developing stomach cancer, it also decreases the risk of developing asthma. Researchers found that H. pylori modulated immune cells known as dendritic cells such that they did not activate an aggressive immune response but instead activated what is known as a tolerogenic immune response, which ultimately protects against allergic asthma.
Journal Of Clinical Investigation, Feb 2012