Placing 'immunologic' as an adjective before a collection of lung diseases implies that the host has responded to the illness by reacting with distinctive airway and/or alveolar space cells and proteins, i.e. lymphocytes, macrophages, inflammatory cells, immunoglobulin (antibodies), possibly complement components and various cytokine mediators. Also implicit in the usage of the term is that certain research methods have been used to study the host response. Direct immunologic investigation of the human lung has advanced greatly by the use of fiberoptic bronchoscopy, available for the past 20 years, which has permitted sampling of the airways and alveolar space with bronchoalveolar lavage (BAL). Values for immune components in BAL have been established for normals and used to contrast abnormal findings in many forms of lung disease. Examples of these findings in pulmonary sarcoidosis and hypersensitivity pneumonitis are given. In contrast an immunologic deficiency, as may be found with acquired immunodeficiency syndrome, illustrates how a lack or imbalance in immune components can contribute to infection and chronic disease. Overall, the analysis of immune components in BAL fluid has contributed to improved concepts about the immunopathogenesis of many lung diseases.