Pathogenesis

Eosinophils are derived from undifferentiated myelocytic bone marrow cells and reside primarily in tissue. Normally, eosinophils are several hundred-fold more abundant in tissues than blood, and are most numerous in tissues having an epithelial interface with the environment such as the respiratory, gastrointestinal, and lower genitourinary tracts. Cytoplasmic granules, called specific granules, are responsible for the distinctive staining properties of eosinophils seen by light microscopy. Specific granules have a crystalloid core composed of major basic protein which is toxic to helminthic parasites, tumor cells, and host cells. Eosinophils are not found in the lungs of normal individuals, thus, their presence in biopsy specimens and BAL fluid signals a pathologic process.

AEP is associated with a variety of clinical disorders including fungal and parasitic infections, immunologic and systemic diseases, and drug toxicity. Less commonly, AEP has been reported with initiation of cigarette smoking, and inhalation of dust, Scotchguard, heroin, and smoke from fireworks. Although many associations are known, the exact cause or causes, and mechanism by which eosinophils are recruited into the lungs in AEP are unclear.