Nature
One fish is immune to a sting that kills almost everything else that touches it
Sea anemones sting and paralyse fish that brush against their tentacles, but clownfish live inside them unharmed, coated in a layer of mucus that stops the anemone's stinging cells from firing. In exchange for shelter and protection from predators, the clownfish drives off the anemone's own predators and parasites, and its waste may even feed the anemone. Neither species does especially well without the other.