Immunologic lung disorders are accompanied by an array of laboratory abnormalities, some of which contribute to disease pathogenesis. Allergic bronchopulmonary aspergillosis, which complicates asthma and cystic fibrosis, causes mucous plugging of airways, eosinophilic pneumonia, and bronchiolitis obliterans. Aspergillus fumigatus, growing saprophytically in bronchial mucus, is responsible for most cases, and prednisone, not antifungal agents, is the drug of choice because it controls the immunologic responses of the lung. In cystic fibrosis, epithelial surface fluid from the lung does not kill Pseudomonas aeruginosa, in part because antibodies to P aeruginosa are plentiful but ineffective in opsonizing bacteria. Neutrophil-derived elastase cleaves immunoglobulins and digests the C3b receptor on neutrophils, which limits phagocytosis of pathogens. In helminth infections and infestations, pulmonary and peripheral blood eosinophilia can be accompanied by increases in total and antiparasite IgE concentrations and generate TH2 CD4+ T-lymphocyte responses. Understanding the immunologic abnormalities of lung disorders may lead to more effective therapies.