The molecular events in heavy chain class-switching.

Heavy chain class-switching is the process by which B lymphoid cells change the constant region of the immunoglobulin heavy chain they produce. Class-switching is most commonly accomplished by recombinationldeletion between switch recombination regions that lie upstream of each germline heavy chain constant region gene. Recent studies support a model that recombination to specific switch regions is directed by modulation of the accessibility of these regions to a common class-switch recombination system. T cell lymphokines seem to be able to alter the accessibility of different heavy chain constant region loci, and thereby direct the specificity of class-switch recombination in B cells.