Insects, worms, and vertebrates all belong to the "triploblast" or "Bilateria" clade of metazoans (Fig. 1). Since several animal phyla diverged, however, before the origin of this clade, the discovery of shared molecules tells us little about their origin and original roles, until we have comparative data from more basal animals. The aim of this chapter is to review the experimental evidence for innate immune reactions in Cnidaria, an ancient group of animals which diverged long before insects and nematodes were around (Fig. 1). Although cnidarians have a long history as model systems in comparative immunology (Campbell and Bibb 1970; Du Pasquier 1974, 2001), the underlying molecular mechanisms are poorly understood. Here
I summarize 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.
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