Fish B lymphocytes: Defining their form and function

Fish B lymphocytes have been defined, as in mammalian systems, as those lymphocytes that express immunoglobulin on their surface and secrete specific antibody in response to antigenic stimuli. Such cells have been identified in a number of fish species. However, the physical characterization of these cells, their distribution, the form of their response, and the antibody product itself have not precisely followed the mammalian paradigm. The implications of these unique features of fish B cell form and function are the focus of this review. The antigenic phenotype of the B cell and the possible function of distinct subpopulations are discussed. The origin of the fish B lymphocyte itself is still a great enigma. The developmental and ontological pathway(s) taken by fish B cell progenitors are still virtually unknown, but considerable data have accrued that allow investigators to postulate possible sources of such lymphocytic stem cells (i.e. bone marrow equivalents), such as the anterior kidney. As opposed to the ontological development of B cells, the nature of the intercellular cooperation and lymphokine requirements for an antibody response seem to parallel mammalian counterparts quite closely. Finally, the coordination of all these cells, factors, and products into a complex function, such as the generation of immunological memory, present an excellent venue for comparative immunologists. In the expression of this complex and highly regulated process, fish have demonstrated some basic similarities to mammals, but differ in other important features. Understanding how fish have circumvented the need for these aspects of memory allows us not only to understand fish function, but enables us to place specific mammalian functions in a broader context. Such analyses will provide new theoretical approaches to the understanding of mammalian as well as fish immune responses.