Background Symptoms of allergic rhinitis are accompanied by infiltration of the nasal mucosa with inflammatory cells, predominantly eosinophils and metachromatic cells (basophils and mast cells). Specific immunotherapy (IT) reduces mucosal eosinophilia and numbers of metachromatic cells in the epithelium. A specific marker distinguishing basophils from mast cells was recently developed.

Objectives The basophil‐specific monoclonal antibody 2D7 was used to determine the influence of subcutaneous IT on numbers of nasal mucosal basophils compared with the effects of IT on neutrophils, eosinophils and mast cells.

Method During a randomized, placebo‐controlled trial of grass pollen IT in 44 adults with severe summer hay fever, nasal biopsies were taken at baseline, out of the pollen season, and at the peak of the pollen season following 2 years treatment. Biopsies were processed for immunohistochemistry for basophils (2D7⁺), mast cells (AA1⁺), eosinophils (MBP⁺) and neutrophils (neutrophil elastase⁺).

Results In placebo‐treated (PL) patients there were significant seasonal increases in basophils (P < 0.01), mast cells (P < 0.05) and eosinophils (P = 0.002) in the nasal submucosa. In IT‐treated patients significant increases in 2D7⁺ cells (P < 0.01) and eosinophils (P = 0.01) but not AA1⁺ cells (P = 0.9) were observed. These differences were significant between groups for eosinophils (P < 0.05). In the epithelium there were seasonal increases in AA1⁺ cells and eosinophils in both groups (PL: P < 0.01, IT: P < 0.05 for both). The between‐group difference was significant for eosinophils (P = 0.05). Basophils were observed in the epithelium of six out of 17 in the placebo group and one out of 20 in the IT group (P = 0.03). Neutrophil numbers remained constant in both epithelium and submucosa.

Conclusion Successful grass pollen immunotherapy was associated with inhibition of seasonal increases in basophils and eosinophils, but not mast cells or neutrophils within the nasal epithelium. Immunotherapy may act, at least in part, by reducing seasonal recruitment of basophils and eosinophils into the epithelium.