The lungs are exposed to a wide array of inhaled particles and organisms. Their ability to cope with noxious agents is extremely efficient, providing that the various protective mechanisms are not too severely disturbed. The first line of defence is mechanical, followed by innate immunity, which takes a stepwise approach to prevent undue inflammation and gas exchange disturbances. If antigenic particles reach the layers beyond the epithelial lining an adaptive immune response will be organised by an amazing traffic of antigen-presenting cells into the hilar lymph nodes [1]. Memory T-cells will then come back to the lungs, where they can be triggered to further multiply. These mechanisms will be reviewed in the following series of articles.
Recently, it has been learnt how pathogens may influence the adequacy of the immune system to fight viruses, other intracellular pathogens or bacteria early in life. It is becoming clear that the environment may condition some individuals at a young age to be particularly …