The freshwater bivalve, Corbicula fluminea, was selected for the study of the internal defense mechanisms of hemocoelic invertebrates. Corbicula hemocytes, the primary effector cells involved in defense reactions, consisted of a heterogeneous cell population which could be divided into three groups; namely small hyaline hemocytes, vesicular hemocytes, and large granular hemocytes. All hemocytes were capable of recognizing and phagocytizing, in vitro, formalin-fixed vertebrate RBCs. This phagocytic response was dependent on a heat-labile opsonin-like component(s) present in clam plasma. In addition, hemocytes, as well as clam plasma, contained a cytolytic substance which was reactive against a variety of mammalian RBC targets. Similarities in the physico-chemical and functional properties of the hemocyte and plasma lysins suggested that these were the same molecule and that hemocytes may represent an important source of circulating plasma lysin. Plasma alone was capable of agglutinating several species of fresh and formalized vertebrate RBCs.Based on the in vitro studies, it has been shown that Corbicula is able to recognize and react against foreign particles through both cellular and humoral mechanisms. The functional relationship between cellular and humoral components has been demonstrated in the hemocyte phagocytosis study. Additionally, hemocytes represent a major source of the plasma lysin
