Pulmonary eosinophilic infiltrates

Pulmonary eosinophilic infiltrates include an heterogeneous group of disorders characterized by the presence of eosinophils in the lungs as detected by bronchoalveolar lavage or tissue biopsy, with or without blood eosinophilia. The disease can be idiopathic (simple pulmonary eosinophilia, acute and chronic eosinophilic pneumonia, hypereosinophilic syndrome), secondary (to drugs, parasites, fungal and mycobacterial infection, irradiation, toxic products) or associated with diffuse lung diseases (connective tissue diseases and some neoplasms). Pathologists faced with eosinophils in the lungs (either on cytology or biopsy) should keep in mind several possibilities, although a diagnosis of certainty is rarely based on morphology alone. Correlation with laboratory tests, imaging studies and clinical presentation has a key role, even if some pulmonary eosinophilic diseases are sufficiently characteristic on clinico-radiologic ground to not require a biopsy (e.g. some drug reactions, parasitic infections, idiopathic hypereosinophilic syndrome, allergic bronchopulmonary aspergillosis). Nevertheless, pathologists can play a central role because they can be the first to note eosinophils in the lungs of a very sick patient. Knowledge of histologic features and a striking collaboration with other physicians are necessary to achieve correct diagnosis and to establish adequate treatments.