The nonspecific immune system of the lung

The lung possesses two immune systems, one in the alveolar space, which consists primarily of alveolar macrophages, and one in the interstitium and the bronchus-associated lymphoid tissue (BALT), which comprises both nonspecific and specific components. The primary targets for invading foreign material are the alveolar macrophages. These secrete in response to stimulation chemotactic substances which attract within a short time granulocytes into the alveoli. The alveolar macrophages are especially well equipped with defence mechanisms against microorganisms. Material which passes this first nonspecific barrier enters the interstitium, where it is phagocytosed and presented to the interstitial lymphocytes. Antigen presentation and T-cell recognition also takes place in the BALT. Interstitial macrophages are particularly well equipped with mechanisms that are important for antigen presentation, such as expression of MHC 2, IL-1 and IL-6 secretion.