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BACKGROUND: Eosinophils are thought to play an important role in the symptomatology and pathophysiology of allergic rhinitis. Most quantitative studies on eosinophils in nasal mucosa have focused on the dynamics of eosinophils in the acute and late phases of the allergic reaction by using different cell sampling techniques. Little is known about the dynamics of eosinophils during a more prolonged period of allergen exposure and the activation of eosinophils induced by allergen challenge. OBJECTIVE: The aim of this study was to investigate the dynamics and activation of the eosinophils in the nasal mucosa of patients with an isolated grass pollen allergy during an out-of-season 2-week allergen exposure, mimicking the natural grass pollen season. METHODS: Seventeen patients with isolated grass pollen allergy and four control subjects were challenged daily with the allergen during a 2-week period in the winter. Nasal brush specimens were obtained before provocation and each day during the provocation period. Biopsy specimens were obtained once before, six times during, and once after the provocation period. Preparations made of nasal brush and nasal biopsy specimens were stained with the monoclonal antibody BMK 13 and Giemsa stain as paneosinophil markers and with the monoclonal antibody EG2 to identify activated eosinophils. RESULTS: We found significant increases in the total number of eosinophils and the number of activated eosinophils in the epithelium and lamina propria. These increases were most explicit in the second week. BMK 13 was found to be a paneosinophil marker superior to Giemsa staining. CONCLUSION: Eosinophils are not only involved in the acute and late phases of the allergic reaction but are probably even more involved in the chronic phase