The sea urchin immune system

Metchnikoff’s use of sea star larvae to observe encapsulation and phagocytosis, which was followedmuch later by allograft rejection kinetics, revealed that echinoderms had an innate immune system thatwas lacking of adaptive attributes. Larval sea urchins mount defenses in response to contact withmicrobes, which are mediated by phagocytic blastocoelar cells and pigment cells. In the adult, thecoelomocytes mediate immune responses through phagocytosis and encapsulation of foreign particles inaddition to degranulation of antimicrobial molecules. Molecular analysis of immune functions in the seaurchin has demonstrated a complement system that appears to have multiple alternative pathways andseveral activators of the lectin pathway, but may be missing the terminal pathway. Other genes andproteins involved in the sea urchin immunity include expanded sets of lectins, proteins with scavengerreceptor cysteine-rich repeats, Toll-like receptors and associated signalling proteins. A vast array ofproteins belonging to the 185/333 family are expressed in coelomocytes in response to lipopolysaccharideand show a surprising level of diversity. The sea urchin innate immune system has a number of largegene families with unexpected complexities and elevated levels of diversification.