It was long believed that acquired immunity--a type of immunity mediated by T- and B-cells--had memory, meaning that it could learn from new pathogens, making subsequent reactions more effective, whereas innate immunity--which is mediated by macrophages and other types of cells that react to certain molecules typically associated with pathogens--did not. However, it gradually became clear that things were not so simple. Plants and insects, which only have innate immunity, also seem to have immunological memory. Further, it has been reported that herpes virus infection increases the resistance against bacteria in vertebrates. These phenomena suggest that innate immunity also has memory, but researchers have been reluctant to accept the hypothesis given the lack of a mechanism Now, in research published in Nature Immunology, a research team led by Keisuke Yoshida and Shunsuke Ishii of the RIKEN Molecular Genetics Laboratory has revealed that epigenomic changes induced by pathogen infections, mediated by a transcription factor called ATF7, are the underlying mechanism of the memory of innate immunity.