In the case of the heavy chain... In naive,mature B cells,the primary mRNA transcript contains VDJ and BOTH C and Cs. This primary transcript can be differentially processed to give rise to mRNA encoding either IgM or IgD. This explains why naive B cells express both IgM and IgD. The variable region of both are encoded by the same VDJ-so they have the same antigen specificity. In the case of the heavy chain… In naive, mature B cells, the primary mRNA transcript contains VDJ and BOTH Cm and Cd . This primary transcript can be differentially processed to give rise to mRNA encoding either IgM or IgD. This explains why naive B cells express both IgM and IgD. The variable region of both are encoded by the same VDJ - so they have the same antigen specificity