Pulmonary alveolar proteinosis (PAP) is a rare lung disorder characterized by alveolar filling with floccular material derived from surfactant phospholipids and protein components. PAP is described as primary or secondary to lung infections, hematologic malignancies, and inhalation of mineral dusts. Recently, the importance of granulocyte/macrophage colony-stimulating factor (GM-CSF) in the pathogenesis of PAP has been documented in experimental models and in humans. GM-CSF signaling is required for pulmonary alveolar macrophage catabolism of surfactant. In PAP, disruption of GM-CSF signaling has been shown, and is usually caused by neutralizing autoantibodies to GM-CSF. Therefore, the emerging concept is that PAP is an autoimmune disorder resulting in macrophage and neutrophil dysfunction. In a recent report, it has been reported that GM-CSF autoantibodies are normally present in healthy individuals, but at lower levels than in PAP patients [171] . In addition, in vitro experiments indicated that these autoantibodies reduce GM-CSP signaling similarly in healthy individuals and in PAP patients. At levels above a critical threshold, GM-CSF autoantibodies are associated with multiple impaired GM-CSF dependent myeloid function [172] . Several cases of genetic defects in the common beta chain for the GM-CSF receptor have been documented [173] . Eosinophilic lung diseases Eosinophilic lung diseases constitute a diverse group of disorders of various origins. The diagnosis is suggested by the presence of pulmonary infiltrates on chest imaging and peripheral eosinophilia. It is confirmed by the presence of increased amounts of eosinophils in BAL and/or lung tissue eosinophilia. In this section, eosinophilic vasculitis will not be discussed (see chapter 6.2.2). The search for an etiology includes a combination of clinical and laboratory investigations. Eosinophilic lung diseases of known cause in children include mainly allergic bronchopulmonary aspergillosis, parasitic infections and drug reactions. Eosinophilic lung diseases of unknown cause comprise Loeffler syndrome (characterized by migrating pulmonary opacities), acute eosinophilic pneumonia, and chronic eosinophilic pneumonia [174, 175] . The idiopathic hyper-eosinophilic syndrome is a rare disorder observed mainly in adults; it is characterized by prolonged eosinophilia and a multiorgan system dysfunction due to eosinophil infiltration with pulmonary involvement documented in almost half of the patients [176, 177] .