Ancient immunity. Phylogenetic Emergence of recognition-defense mechanisms

Although still scarcely considered by the majority of the biomedical world, invertebrates have greatly contributed to the elucidation of fundamental biological problems. Let us think to the birth of cellular immunology with Meˇcnikov’s experiment on sea star larvae, or the importance of squids for the comprehension of the molecular basis of nerve conduction, or the contribution of sea urchins to the studies of fertilization and early development. Indeed, invertebrates represent the overwhelming majority of animal biodiversity and this simple assumption can explain the growing interest of the scientific community towards various aspects of their biology. Invertebrate immunobiology is one of such aspects. Invertebrate immunity is of the innate type, which means without the somatic recombination events occurring in lymphocytes of jawed vertebrates and accounting for the high specificity of their responses. However, even if lacking the adaptive branch, this does not mean that invertebrate immune systems are as simple as originally thought. Indeed, the huge variety of adaptations led to a high diversification of the responses, and this Special Issue, including two research papers and six reviews, presents some examples.