Structural studies of monoclonal immunoglobulins demonstrated that immunoglobulin chains are organized in the antigen-binding variable (V) part and the constant (C) part. The variability pattern indicated the separate genetic control of V and C parts. The decisive steps in B cell differentiation are gene translocation and fusion events which assemble V and C genes to the actual one active H chain gene and the one active L chain gene in a B cell or plasma cell. Further gene translocations, now solely on the H chain C genes, effect the H chain class switch. By DNA structure analysis of immunoglobulin genes from stem cells and plasma cells, the details of the internal organization of the genes and of the gene translocations were recognized. RNA processing of H chain precursor mRNA decides whether the immunoglobulin molecule is produced as an antigen receptor or an antibody molecule. It also enables the simultaneous expression of antigen receptors as IgM and IgD molecules on the B cell membrane.