The Path Less Explored: Innate Immune Reactions in Cnidarians

The phylum Cnidaria is one of the earliest branches in the animal tree of life. Cnidarians possess most of the gene families found in bilaterians and have retained many ancestral genes that have been lost in Drosophila and Caenorhabditis elegans. Characterization of the innate immune repertoire of extant cnidarians is, therefore, of both fundamental and applied interest — it not only provides insights into the basic immunological “tool kit” of the common ancestor of all animals, but is also likely to be important in understanding human barrier disorders by describing ancient mechanisms of host/microbial interactions and the resulting evolutionary selection processes. The chapter summarizes four aspects of immunity which can be studied particularly well within cnidarians — and which may be of interest from a comparative point of view to all immunologists: intraspecies competition in sea anemones, allorecognition and cell lineage competition in the marine hydrozoan Hydractinia, antimicrobial defense reactions in Hydra and jellyfish, and symbiotic relationships in both corals and Hydra. Studies in cnidarians reveal that there is no problem in innate immunity these basal metazoans did not attempt to solve. Thus, whatever we experience with our own innate immune system, whatever we hope to learn, we will see that the cnidarians have been there before us.