Microalgae of the dinoflagellate group have engaged in intracellular symbioses with corals since primeval times. Researchers from the Centre for Organismal Studies of Heidelberg University recently discovered that such symbioses depend on the ability of the algae to suppress the immune system of their host cell and thereby avoid being “spit out” again. The researchers also found indications that this cellular immune response is an evolutionarily ancient mechanism that is more widespread than previously assumed.