The lungs are protected from infection by several natural antiviral and antimicrobial molecules. The precise mechanisms of action of most of these native anti-infectives have not been elucidated yet. It has been observed that the activities of these molecules are impaired in inflammatory airway diseases, lung infections and cystic fibrosis. By developing and using model systems of lung epithelia we hope to determine how the combined activities of different host defence molecules affect immune cells and epithelial cells and help to protect against infections. Alternatives to antibiotics have been developed to fight infections in patients with cystic fibrosis.