Final answer:
The mechanism allowing Ig to be synthesized in either a membrane-bound or secreted form is alternative RNA splicing, which results from the differential processing of the primary RNA transcript of the Ig gene, leading to different outcomes - either a membrane-bound receptor or a secreted antibody.
Step-by-step explanation:
The mechanism that permits immunoglobulin (Ig) to be synthesized in either a membrane-bound or secreted form is alternative RNA splicing. This process allows a single gene to produce two different forms of Ig. On one hand, B cells at the early stage express Ig as a B cell receptor (BCR) on their surface, which is a membrane-bound monomer for IgD and IgM. On the other hand, after encountering the antigen, B cells can differentiate into plasma cells that secrete Ig. The alternative RNA splicing of the primary RNA transcript from the Ig gene switches the production from the membrane-bound form to the secreted form of the antibody.
For example, IgM is initially produced in a membrane-bound monomeric form on naïve B cells. After activation, B cells can switch to produce a secreted form of IgM, which assembles into a pentamer with a J chain, enabling it to bind to many antigens simultaneously making it a key antibody in immune responses. Secretory vesicles then transport these secreted Igs outside the cell.