ABSTRACT
Maturation of the antibody repertoire consists of two successive steps. The first leads to the generation of the primary repertoire composed of immunoglobulin M (IgM) with a low affinity for antigen. It occurs in primary lymphoid organs (fetal liver and bone marrow) in an antigen-and T cell-independent manner. It is acquired by stochastic rearrangements of the different genetic regions of the immunoglobulin gene (rearrangement between the variable (V) region and the constant (C) region encoding the IgM isotype). The V region recognizes specifically the antigen, and the C region determines the isotype of the immunoglobulin (IgM). Mature B cells, therefore, express on their membranes IgM as the B-cell receptor (BCR), and can produce only IgM.
