Nick Gay and his collaborators have published a paper in PNAS entitled . The ability of organisms to detect and respond to infection by microorganisms is fundamental and has ancient evolutionary origins. In mammals, immune system cells recognize danger molecules directly using pattern recognition receptors which often belong to the Toll family. In insects, it has been found that Gram-positive bacteria and fungi can also be detected indirectly by an endogenous molecule, Spätzle (Spz), that activates related Toll receptors, leading to an effective immune response. In this work, the group has determined the molecular structure of a Spz/Toll complex, which reveals that Spz’s mode of action is similar to neurotrophins, a family of proteins involved in the development and homeostasis of the insect and vertebrate nervous systems.