SUMMARY. The lung is directly and continuously exposed to the environment and has therefore developed strong defense mechanisms, which, as those of other organs, include innate and adaptive immune responses. Mechanical barriers, secretions of the bronchial mucosa, antimicrobial constituents of the blood, such as the complement system, and the leukocytes and phagocytes of the pulmonary system are all part of the innate defense. Adaptive immunity is deployed through delayed mechanisms, which are mediated mainly by T-cells, B-cells and the antibodies they produce. Adaptive immunity is specific for each virulent factor; it leads to the development of immune memory and therefore leads to successful and rapid response against that specific pathogen in future encounters. The balance between all these immune mechanisms is crucial for the deployment of a successful defense against infectious agents, cancer cells and autoimmune disorders, and for minimization of collateral lung tissue damage. Disorders of immune response may occur, leading either to reduced response and immune deficiency (serious infections) or to overreaction of the immune response, with allergy or autoimmune disease. Such disorders can characteristically be observed in diseases such as bronchial asthma, pulmonary emphysema, granulomatous inflammation, idiopathic pulmonary fibrosis and acute respiratory distress syndrome. Pneumon 2007; 20(3):274-278