Molluscan immune defenses.

The interest of marine invertebrates as food resources provides a major interest to study molluscan immunity for better understanding of the host response to pathogens. Molluscs possess a natural immunity formed by anatomical and chemical protective barriers that prevent damage of the underlying tissues, body fluid losses and the infections of pathogenic microorganisms and parasites. The main physical barrier is shell and mucus which cover the soft body of molluscs. The integrity of body coverings is supported by blood clotting and wound healing. The internal defense mechanisms of molluscs involve such cellular reactions as: phagocytosis, nodule formation, encapsulation, pearl formation, atrophy, necrosis and tissue liquefaction. Granular hemocytes are the most numerous cell type of molluscan blood active in cellular defenses. Invaders small in size are eliminated by phagocytosis in which participate lectins and products of prophenyloxidase system activation. Numerous and large intruders are eliminated by nodule formation or encapsulation, either cellular or humoral. Humoral components of molluscan immunity are formed by lysozyme activity, lectins and the phenyloxidase system. Up to now the role of mercenenes, paolins, acute phase reactants, alpha 2-macroglobulins and multifunctional binding proteins with anti-protease activity is not well clarified yet. Research prospects on the field of molluscan immunology should essentially be devoted to study cellular defense functions and humoral effectors to select pathogen-resistant molluscs. This aim could also be achieved by the identification and characterization of immune genes which are candidates for molluscs genetic transformation.