The echinoderm innate humoral immune response

Abstract Multicellular organisms have an immune system, which is essential for the survival of living beings. Interest in the immune system has been expanded since common characteristics of innate immunity between Drosophila melanogaster (Meigen, 1830) and mammals were discovered in the 1980. Since then, immunology has mainly focused on the adaptive immune system that seems to be restricted to vertebrates. Unlike the innate immunity, the adaptive one is acquired after exposure to a specific antigen (Ag) and includes: antigen-presenting cells such as macrophages, proliferation of B and T lymphocytes, Ag-specific antibody/cytokine production and immunological memory. Innate immunity is instead a process of cellular defense at low specificity, which is designed to prevent and combat infectious agents that penetrate at the tissue level, and may be the only form of immunity present in invertebrates such as sea urchins. The immune system of invertebrates acts through (i) cellular components (cell-mediated immunity) in which the effectors of defense reactions are represented by immune cells; (ii) soluble factors (humoral immunity), secreted by the immune cells, such as lectins, agglutinins, lysins, antimicrobial peptides and the prophenoloxidase (proPO) activating system, which act in parallel with the immune cells to fight pathogens and other foreign substances. Here we aim to deepen the study on humoral immunity of invertebrates, especially referring to the phylum Echinodermata because of its features shared with protostomes and other deuterostomes, and suggesting a key step during evolution.