The humoral immune response is the culmination of a complex series of cellular interactions and migrations that define specific pathways of antigen-driven B cell differentiation. There are about 5 x 10{sup 8} and 10{sup 12} B lymphocyte lineage cells in the mouse and human, respectively, after the immune system has been established. B lymphocytes perceive antigen in their environment by virtue of their surface receptors (B cell receptor; BCR), in which membrane-associated immunoglobulin (mIg) is the antigen recognition substructure and the associated Ig{alpha} and Ig{beta} molecules transduce the activation signal. mIgs have extensive structural homology to immunoglobulins which are secreted by differentiated daughter cells following antigen stimulation. The specificity of a given BCR or antibody is created by a programmed successive process of gene rearrangements, first of D to J segments, then of V to DJ segments of the Ig heavy (H)-chain gene loci, and finally, of V to J segments of the Ig light (L)-chain gene loci. V,D, and J elements are assembled in a enormous number of combinations; variability is further achieved at the break-points of joining segments, including the insertion of non-germline-encoded nucleotide (N)sequences at the borders of V(D)J points, hence the almost unlimited specificities of themore » B cells or antibody repertoire. 129 refs.« less