Final answer:
The antigen-binding sites of IgD and IgM membrane immunoglobulins are the same in structure and function, with both involved in binding to specific antigens on B cells.
Step-by-step explanation:
Both IgD and IgM membrane immunoglobulins are found on the surface of B cells and facilitate antigen recognition through their Fab antigen-binding sites. Specifically, the Fab regions of these immunoglobulins are responsible for binding to specific antigen epitopes. Regardless of whether the immunoglobulin is IgD or IgM, the structure of the Fab region and its antigen-binding capability are the same. IgM is often found in its pentameric form upon secretion, with ten antigen-binding sites available, making it effective in binding multiple antigens simultaneously. Though IgD is also capable of binding antigens, it is chiefly involved in the maturation of B cells and is present in minimal quantities in serum compared to IgM.