Eosinophils in eosinophilic pneumonia.

Idiopathic eosinophilic pneumonia (IEP) is an uncommon disease of unknown aetiology characterized by the accumulation of eosinophils in the lung, variable blood eosinophilia and peripheral opacities on the chest radiogram. The response to steroid treatment is dramatic but relapses are frequent, requiring the use of long-term steroid treatment. Eosinophils are primarily tissue cells, containing in their granules many preformed proinflammatory and cytotoxic mediators. They are also able to synthesize many other substances that defend the host against exogenous stimuli ; under normal conditions the eosinophil number in those tissues that have contact with the environment is 200–400 times greater than in the blood . An abnormally high accumulation of these cells within the lung, frequency reaching up to 50% of the total alveolar cell population , is the hallmark of IEP, but the role of the eosinophil in the immunopathogenesis of lung damage in this disorder is still not completely elucidated. Eosinophils accumulated within both interstitium and small air spaces are hypodense and widely degranulated cells. They show typical morpho-functional characteristics of activated cells and are able to damage parenchymal lung structures by releasing a variety of toxic substances. Consequently, two major points need to be investigated and discussed: 1) how do eosinophils cause parenchymal injury? 2) how are eosinophils recruited and activated at the site of inflammation?