Parasitoid wasp
In endoparasitoids, host immune cells can encapsulate the eggs and larvae of parasitoid wasps. In aphids, a secondary endosymbiont, Buchnera aphidicola carries a latent phage. This phage makes the aphid relatively immune to their parasitoid wasps by killing many of the eggs. However, wasps counter this by laying more eggs in aphids that have the endosymbiont so that at least one of them can hatch and parasitize the aphid.