ABSTRACT
In order for B and T lymphocytes to respond specifically to immune challenge, only a single species of antigen receptor is expressed per cell. In cartilaginous fishes, such as sharks and skates, there can be >100 immunoglobulin (Ig) gene clusters, a situation in which regulatory 200processes permitting expression of only one allele of one cluster are not well understood. This chapter reviews the shark IgM heavy (H) chain gene organization, H chain rearrangement, and recombination events occurring in single B lymphocytes. In order to gain some insight into how the process may be controlled at multiple loci in B cells, somatic rearrangements at IgH genes in thymocytes and deduced recombination events in germ cells are examined. Studies in the shark demonstrate that certain mechanisms deemed crucial to V(D)J recombination in mammalian IgH systems—two-stage rearrangement, locus compaction—are processes that contribute to, but in themselves are not unconditionally required to bring about H chain exclusion.
