AbstractIgG antibodies and eosinophils kill schistosomula ofSchistosoma mansoni in vitro, and there is now evidence to suggest that the main factor that contributes to the expression of purified IgG effector function is the degree of activation of the donor's eosinophils. This study was designed to identify serum‐derived activating factors in sera from individuals infected withS. mansoni.Such activating factors may be responsible for enhancing eosinophil cytotoxicity against schistosomula. Serum‐borne mediators were prepared by fractionation of sera from infected individuals by gel filtration high‐performance liquid chromatography. The eosinophil‐stimulating activity of these mediators was assayed by a new method which depends on the increased expression of the CR3 α chain (CD11b) on the surface of activated eosinophils. Sera from infected individuals exhibited different levels of eosinophil activation, and activation appeared to be due to several serum factors, including interleukin 5. In conclusion, our results suggest that eosinophil‐activating factors present in infection sera may not only be responsible for enhancing eosinophil cytotoxicity but also be necessary for its
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