Respiratory system is formed by two components: Lungs (principally responsible for the respiratory function) and chest wall, which allows adequate ventilation. The lungs are constituted by a system of conduction (bronchia and bronchioles) and a system of gaseous exchange (alveoli), assembled by connective tissue and surrounded by the visceral pleura. The alveoli are the structures responsible for the gaseous exchange, located in the wall of the respiratory bronchioles or in the peripheral region of the alveolar sacs. Alveoli are not isolate structures but are found physically joined among themselves, to the bronchioles and to the visceral pleura and connected to other pulmonary structures by Köhn’s pores and Lambert’s canals which communicate the alveolar sacs to the terminal bronchioles. Structurally two cell types form the alveolar region, type I and type II neumocytes. Although the numerical proportion between both is , the type I neumocytes cover an area 25 times greater than that of type II neumocytes. Participation in the gaseous exchange is the principal function of the type I neumocytes, whereas the type II neumocytes are responsible for the production of surfactant and cellular regeneration after aggression to the alveolar region. The alveolar macrophages are the principal defensive element of the alveolointerstitial region. In their surface are expressed receptors related to adhesion (CD11a/CD18, CD29/CD49, CD54), to the capture of antigens joined to immunoglobulins, complement, or proteins (CD14, CD16, CD32, CD64, Cd11b/CD18, CD11c/CD18), to the capture of nutrients (CD71), and related to the response to cytokines (CD25, CD115-130) and molecules related to the antigenic presentation (HLAII). Alveolar macrophages could be activated by different stimuli (e.g., IFN) and their response involved release of lysosomal enzymes, production of free oxygen radical, and generation of nitric oxide.