The immune system maintains the physiological and genetic integrity of living organisms by recognizing, neutralizing, and clearing potentially harmful biotic and abiotic agents. For more than 550 million years animals have evolved a diverse array of immune strategies. Immune mechanisms in most invertebrate taxa, however, remain largely unknown. The emergence of infectious diseases has caused a rapid decline in various marine ecosystems, especially in coral reefs. As in any infectious disease, the health/disease balance is the result of a complex network of interactions between microorganisms, the environment, and the host immune system. In the last decade there has been increasing efforts in characterizing microorganisms associated to cnidarian diseases, as well as in determining local and global environmental factors related to mortality. However, the mechanisms of immune competence of cnidarians have not been fully addressed. The colonial cnidarian Hydractinia symbiolongicarpus is a recognized animal model in several biological disciplines, including comparative immunology, and methods for maintaining and propagating it are well established. This model system is the ideal choice to study the immune repertoire in cnidarians. Here we propose to characterize the immune transcriptome of H. symbiolongicarpus after immune challenged by using next-generation sequencing technologies coupled with RACE and qPCR strategies. Characterization of immune-type molecules in this model will allow straightforward identification of homologous proteins in other cnidarians proved susceptible to infectious diseases, such as reef-building corals and octocorals. This in turn will contribute to a better understanding of the critical state of corals worldwide, and to take immune-based measures to mitigate such problem. In addition, the results of this project will be published in an indexed journal and the project will constitute the thesis of at least two graduate students.