For most organisms, the correct functioning of the immune system is essential to deal with pathogens and parasites. However, immune activation does not come without a cost. Indeed, immune activation is expected to trade off different reproductive components, potentially leading to accelerated reproductive senescence and even affecting the life history trajectories of offspring. We have recently investigated these questions in male field crickets (G. bimaculatus) using a combination of experimental approaches. Results from these long-term studies indicate that immune system activation reduces male investment in some but not all reproductive components. Immune-challenged males reduced their investment in mate attraction (i.e. calling effort) and courtship behaviour over time but did not alter their sperm production. Curiously, although sperm production was unaffected, immune-challenged males prioritised their resources for sperm protection, resulting in reduced accumulation of sperm damage, including DNA, and increased fertility. In addition, we found strong evidence that such immune-induced changes in the male germ line integrity have both short- and long-term intergenerational effects. The young offspring of immune-challenged males developed higher levels of activity and exploratory behaviour (i.e. bolder personalities), behavioural changes that even persisted into adulthood and led to different ageing trajectories. Overall, our findings highlight how immune activation can accelerate the decline of some reproductive traits, but suggest that males may respond to such reproductive costs by prioritising germline protection, with long-term consequences for offspring life-history trajectories. Furthermore, our results highlight the complex interplay between immune function, reproductive senescence and offspring phenotypes, providing valuable insights into evolutionary dynamics.