The knowledge of the immunity in annelids started with the use of earthworms as biomarkers indicating changes caused by environmental pollution. Defence strategies effectively protect earthworms against bacterial infections and parasitic invasion. A natural immunity formed by anatomical and chemical protective barriers prevents damage of the underlying tissues, body fluid losses, and microbial infections of the body cavity. The internal defence mechanisms of annelids involve phagocytosis, nodule formation and encapsulation, blood coagulation and wound repair, and antibacterial immune proteins. The antibacterial activity of coelomic fluid associated with lysozyme-like substances and inducible humoral molecules support haemocytic reactions in the annelid defence system.
Advances in biochemistry and molecular biology have made it possible to identify a number of mechanisms active in the immune phenomena of echinoderms. It is obvious that echinoderms have the ability to distinguish between different foreign objects (pathologically changed tissues, microorganisms, parasites, grafts) and to express variable effector mechanisms which are elicited specifically and repeatably after a variety of non-self challenges. The molecular and biochemical basis for the expression of these variable defense mechanisms and the specific signals which elicit one type of effector mechanism are not, however, yet well known. The high capacity of coelomocytes to phagocytose, entrap and encapsulate invading microorganisms is a valid immune cell-mediated mechanism of echinoderms. The entrapped bacteria, discharged cellular materials and disintegrating granular cells are compacted and provoke the cellular encapsulation reaction. Moreover, humoral-based reactions form an integral part of the echinoderm defense system against microbial invaders. Factors such as lysozyme, perforins (hemolysins) vitellogenin and lectins are normal constituents of hemolymph, while cytokines are synthesized by echinoderms in response to infection.
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