Bacterial infection induces the innate immune response through cytokines such as interferon γ (IFN-γ). Zebrafish embryos lack adaptive immunity and may provide a simple model to study the innate immune system. However, it is not known whether zebrafish and mammalian cytokines are functionally conserved. Here, Dirk Sieger and colleagues show that two zebrafish isoforms of IFN-γ fight infection with Escherichia coli and the natural fish pathogen Yersinia ruckeri. Like mammals, IFN-γ in the zebrafish embryo regulates genes that are necessary to eliminate infection.

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