RATIONALE: rfhSP-D has been shown to suppress house dust mite and Aspergillus fumigatus-induced allergic inflammation in murine models. OBJECTIVES: We sought to elucidate the effect of rfhSP-D on FcεRI and CD23-mediated grass pollen induced allergic inflammatory responses. METHODS: rfhSP-D, containing homotrimeric neck and lectin domains, was expressed in Escherichia coli BL21 (λDE3) pLysS. PBMCs and sera were obtained from grass pollen allergic individuals (n=27). The effect of rfhSP-D on basophil activation and histamine release was measured by flow cytometry. IgE-facilitated allergen binding and presentation was assessed by flow cytometry. Th2 cytokines were measured in cell culture supernatants. The effect of rfhSP-D on IgE production by B cells when stimulated with CD40L, IL-4 and IL-21 was also determined. RESULTS: rfhSP-D bound to Phleum pratense in a dose- and calcium-dependent manner. Allergen-induced basophil responsiveness and histamine release was inhibited in the presence of rfhSP-D, as measured by CD63, CD203c (P=0.0086,P=0.04205), and intracellular-labelled DAO (P=0.0003,P=0.0148). The binding of allergen-IgE complexes to B cells was reduced by 51%(P=0.002) in the presence of rfhSP-D. This decrease was concomitant with reduction in CD23 expression on B cells (P<0.001). rfhSP-D suppressed allergen-driven CD27-CD4+CRTH2+ T cell proliferation (P<0.01), IL-4 and IL-5 levels (all,P<0.01). Moreover, rfhSP-D inhibited CD40L/IL-4 and IL-21-mediated IgE production(77.12%; P=0.02) by B cells. CONCLUSION: For the first time, we show that rfhSP-D inhibited allergen-induced basophil responses at a single cell, level and suppressed CD23-mediated facilitated allergen presentation and Th2 cytokine production. In addition, rfhSP-D inhibited IgE synthesis by B cells, which is also a novel observation.