Women live on average longer than men but have greater levels of late-life morbidity. In addition, some treatments that extend the healthy lifespan of animals in the lab, such as dietary restriction (DR), are more effective in females than in males. What drives these differences between the sexes? We have previously shown that there is a substantial sex dimorphism in the pathology of the aging gut in Drosophila, where females undergo a dramatic deterioration of intestinal integrity, but males do not, and that rescue of this pathology in females underpins the sex bias in the response to DR. We have gone on to show that sexually dimorphic responses to lifespan-extending drugs, such as rapamycin, also benefit females more than males by rescuing their ageing guts. So if gut health is life-limiting for females, what are males dying of? My new lab is seeking to understanding this by studying sex differences in immunity. Women have stronger immune responses than men to most infections but suffer from greater levels of autoimmunity, and this sex difference is apparent throughout the animal kingdom. We use live imaging and the elegant genetics possible in Drosophila to understand how dynamic behaviors of immune cells differ between sexes, and how these affect the response to infection, ageing and survival.