Immunity-related genes that function in cellular and humoral responses are assumed to be under strong selection given their importance in defending against infectious diseases. However, patterns of selection may differ between different components of the immune system, and the strength of selection may vary across populations facing different parasite pressures. Furthermore, the environment in which the host and parasite are embedded can shape defense evolution. Specifically, availability of anti-parasitic resources could relax selection on immune genes. Monarch butterflies (Danaus plexippus), which feed on toxic milkweed plants (Asclepias spp.) as larvae, are distributed worldwide. The milkweed species on which larvae feed strongly affects monarch resistance against a specialist parasite (Ophryocyctis elektroscirrha), with resistance positively correlating with milkweed toxicity. Monarch populations vary in parasite prevalence and in the milkweed species with which they associate. In this study, we examine signatures of selection on sequences of 102 immunity-related genes in four functional classes (recognition, signaling, modulation, and effector) from 37 monarch individuals across four populations. We test if different functional classes exhibit different signatures of selection compared to the genomic background and discuss these patterns of selection in relation to what is known about disease prevalence and host plant availability across these populations.