Microalgae of the dinoflagellate group are known for their ability to survive in other animal cells. These tiny single-cell organisms have engaged in mutually beneficial relationships with corals since primeval times. By passing on critical nutrients to their hosts, dinoflagellates allow corals to thrive even in barren areas. A research team from the Centre for Organismal Studies (COS) of Heidelberg University recently discovered that such symbioses within the cell essentially depend on the ability of the algae to suppress the immune system of their host cell and thereby avoid being “spit out” again. At the same time, the researchers found indications that this cellular immune response is an evolutionarily ancient immune mechanism that is more widespread than previously assumed.