High polymorphism of major histocompatibility complex (MHC) genes that encode proteins crucial for adaptive immunity of vertebrates have been long hypothesized to be maintained by negative frequency-dependent selection resulting from adaptation of parasites to common MHC types, but supporting evidence is scarce. In our new paper in Molecular Ecology (Migalska et al. 2022) we find such evidence, We show that a helminth Aonchotheca annulosa that recently invaded a bank vole population in NE Poland was more likely to infect hosts carrying MHC class II supertypes which where common when the parasite emerged. In contrast, the patterns of frequency dependent may change in time in case of long-established parasites, likely due time-lags inherent to parasite adaptation-host counter-adaptations cycles.