Ig M REF: Jawetz's, Melnick, & Adelberg's Medical Microbiology, 24TH edition chapter 8. Immunology B-cell Receptor for Antigen: B cells express a form of IgM that is located on the cell surface. Cell surface IgM has the same antigen specificity as the secreted IgM antibody molecule. This is achieved by a differential RNA splicing mechanism. The -chain RNA transcript can include a sequence that encodes about 25 hydrophobic amino acids, which enables the IgM molecule to localize in the cell membrane as a trans membrane receptor. Later in development of the B cell, regulation of RNA processing allows expression of a membrane-bound form of IgD, again with the same antigen-binding specificity. Throughout this process, the same V region segment is being expressed with different C region segments.