Review: Overview on the studies of humoral defense factors in bivalve molluscs

Owing to the global development of shellfish culture and increasing awareness of the risks associated with infectious diseases in bivalve molluscs, the importance of studies on defense mechanisms of bivalves has become widely recognized. Japan has a long history of oyster culture, however, there have been meager investigations on diseases and defense mechanisms of bivalves in the country. In foreign countries, especially those in the West, the immunological responses of bivalves, being composed of cellular and humoral factors, have been approarched from various standpoints such as comparative immunology, pathology/health management, environmental monitoring, and exploitation of novel antimicrobial agents. In this paper, studies on humoral defense factors in bivalves represented by oysters, mussels and clams, made mainly with the interest in interaction between hosts and pathogens, are reviewed. For each humoral defense factor, history of researches, chemical and molecular characteristics, and functions of the following factors are presented here with some critical comments: lectins (agglutinins), lysozymes, antimicrobial peptides, pro-phenoloxidase (PO) activating system, and other humoral foctors (protease inhibitors, lysosomal enzymes, heat-shock proteins).