Mature B cells develop in the bone marrow after B progenitor cells have rearranged their v (variable), d (diversity) and j (joining) genes to a functional vdj segment. This happens first for the heavy chains, the µ-chain, resulting in an IgM antibody, then for the light chains. The antibodies that are now formed belong to the so-called natural antibodies.
The second step of development in the lymphatic organs is antigen- and T-cell-dependent. After contact with a suitable antibody by binding to the membrane-bound IgM of the B cell, the B cell proliferates in the germinal centres. This is followed by a change of the immunoglobulin class of the heavy chain ("switch") by "cutting out" the heavy class genes Cµ and subsequent (see below IgE class change) genes up to another immunoglobulin class (Cγ, Cα, Cε), so that IgG, IgA or IgE can be formed. This is followed by the "somatic hypermutation", the over-randomly frequent mutation through base exchange in the gene segments coding for antigen binding. The genetically modified daughter cell produces (and expresses) an antibody with an altered affinity. B cells with the "best" antibody in terms of specificity and affinity are positively selected.