Immunorecognition in Invertebrates with Special Reference to Molluscs

All metazoan animals are protected against invading organisms by an internal defence system. This system comprises different elements. It consists of various cells, tissues and organs which enable organisms to react against potentially harmful pathogens from the external environment and against effete or derailed autologous material. Besides these structural components which form the cellular limb of the immune system diverse humoral factors produced by (several types of) blood cells are engaged in internal defence. In addition to lymphocytes, non-lymphoid cells, especially macrophage- like cells play a crucial role in the immune system (Fig. 1). Macrophages (a) are involved in the induction phase of the immune response (in the recognition of antigenic material and presentation of this material to lymphoid cells; (b) display a regulatory role in the immune response, and (c) play a part in the final phase of the immune reaction (uptake and degradation of antigenic material). The immune system can be divided in a non-specific and a specific part, the latter being characterized by memory formation. Depending on the phylogenetic development of the immune system only few, or more different cell types and humoral factors play a part in the immune response.