AbstractPlatelets can be activated by IgE and are therefore involved in IgE‐mediated effector mechanisms against parasites and in allergic disorders. Here we show that, besides the low‐affinity IgE receptor (FcεRII/CD23), platelets express the high‐affinity receptor for IgE (FcεRI). Flow cytometry analysis revealed the existence of surface FcεRI on platelets with a large heterogeneity among individual donors, and a low proportion of platelets co‐expressing FcεRI and FcεRII/CD23. Northern hybridization and reverse transcription polymerase chain reaction confirmed the presence of mRNA encoding the α, β and γ chains of FcεRI in platelets and in their megakaryocytic precursors. Cross‐linking of FcεRI with monoclonal antibody (mAb) to α chain using either the whole molecule or F(ab′)2triggered platelet cytotoxicity forSchistosoma mansonilarvae. Anti‐FcεRII/CD23 mAb significantly inhibited IgE‐ or FcεRI‐mediated cytotoxicity, indicating down‐regulatory effects of FcεRII/CD23 on FcεRI‐dependent functions. These results demonstrate functional properties for FcεRI on platelets and indicate unsuspected interactions between the low
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