In invertebrates and vertebrates, innate immunity is considered the first line of defense mechanism against non-self material. In vertebrates,cytokines play a critical role in innate immune signalling. To date, however, the existence of genes encoding for invertebrate helical cytokines hasbeen anticipated, but never demonstrated. Here, we report the first structural and functional evidence of a gene encoding for a putative helicalcytokine in Drosophila melanogaster. Functional experiments demonstrate that its expression, as well as that of the antimicrobial factors defensinand cecropin A1, is significantly increased after immune stimulation. These observations suggest the involvement of helical cytokines in the innateimmune response of invertebrates.