Increased numbers of eosinophilic granulocytes that exhibit an activated phenotype are found in bronchial tissue and bronchial alveolar lavage fluid of patients with allergic asthma. Little is known about the processes that lead to activation of eosinophils in vivo, but Igs might be important stimulants. In the present study we investigated the capacity of human eosinophils to interact with beads coated with human serum IgG or IgA. Binding of IgG/IgA-coated beads to eosinophils from normal donors appeared to be dependent on priming with Th2-derived cytokines. Priming with granulocyte-macrophage CSF, IL-4, or IL-5 is required for eosinophils to form rosettes with IgA-beads. IL-4 priming resulted in a fast and transient effect on binding of IgA-beads, whereas the effect of IL-5 priming was slower and longer lasting. The expression of Fc alphaR (CD89) was low compared with that on neutrophils, and experiments with the blocking mAb My43 (CD89) showed no inhibition of rosette formation between eosinophils and IgA-coated beads. However, polymeric myeloma IgA effectively inhibited the rosette formation of IgA-coated beads to eosinophils. Binding of IgG-beads could only be primed with granulocyte-macrophage CSF and IL-5, not with IL-4. These data are concurrent with the hypothesis that Th2-derived cytokines spatially produced at the side of an allergic inflammatory response can direct eosinophils to a rather restricted primed phenotype by IL-4 or to a more generalized primed phenotype by IL-5.